بسم الله الرحمن الرحيم
There is a lot being said on the issue of Shi`i-Sunni unity these days. The view amid discussions on this issue has become such that our Shi`i friends are seen as the reconcilers and tolerant ones, who have always seeked unity and respect between the two groups, and the Sunnis -- or "Wahhabis" if they're up-front -- have always censured and hated them, and still continue to do so. In this context, I just thought of letting our brothers and sisters know that how, through whatever little reading I have done of books by the Ithna' `Asharis [i.e., the vast majority of Shi`is today], I have found our `ulama labelled as Nasibis at every corner by the scholars of the same Shi`is who say that they want to unite with us today.
I will now list a few of those `ulama who I read were labelled as Nasibis by Ithna' `Ashari scholars for brothers to see how the Sunni `ulama -- our elders and predecessors who taught us all that the din of Muhammad (as) that we know -- have been deemed and labelled as the worst of human beings.
I am neither opposing nor accepting unity in this thread. So I would advise all brothers from being judgemental of me. I'm just relaying information, and if anyone disagrees, he has all right to voice his/her difference of opinion and provide evidence to prove those opinions. The conclusion is for the brothers and sisters to reach.
I have divided this short writing into two sections:
[I] The definition and status of a "Nasibi" according to Ithna' `Asharis.
[II] Sunni `ulama labelled as Nasibis by Ithna' `Asharis.
I hope Allah tabaraka wa ta`ala accepts this little effort of mine and that He allows all brothers and sisters to benefit from it.
[I] Definition and Status of a "Nasibi" per Ithna' `Asharis
Ithna' `Ashari scholars have never been hesitant in expressing about the Nasibis and who they are.
al-`Allama al-Hilli says in Nihayat al-Ihkam [1:274]:
والخوارج والغلاة والناصب ، وهو الذي يتظاهر بعداوة أهل البيت ( عليهم السلام ) أنجاس
"The Kharijis, Ghulat and the Nasibi, who is who demonstrates enmity for Ahl al-Bayt `alayhim al-salam, are impure individuals."
When al-Shahid al-Awwal mentions the dog and its "brothers" in his Alfiyya on fiqh, al-Muhaqqiq al-Karaki comments upon it in it -- per his Rasa'il [3:215]:
قوله : وأخواه . أي : الخنزير ، والكافر بأنواعه حتى الخوارج ، والغلاة ، والنواصب ، والمجسمة بالحقية
"His saying: 'Its brothers...' meaning, the swine, and all types of kafirs, even the Kharijis, Ghulat, Nasibis and Mujassima in the true sense."
Muhammad Baqir al-Majlisi says in commentary of a narration in Mir'at al-`Uqul [24:211] where the word "Nasibi" was used for a man:
إن كان المراد بالناصب، المبغض المعاند لأهل البيت عليهم السلام كما هو الأظهر فهو كافر، ودمه هدر
"If by 'Nasibi' it was meant the hater and enemy of the Ahl al-Bayt `alayhim al-salam, which is most obvious, then he is a kafir and his blood is to be spilled unavailingly."
Ni`matullah al-Jaza'iri says about a "Nasibi" in al-Anwar al-Nu`maniyya [2:306]:
الذي ورد في الأخبار أنه نجس وأنه شر من اليهودي والنصراني والمجوسي، وأنه كافر نجس بإجماع علماء الشيعة الإمامية رضوان الله عليهم ، فالذي ذهب إليه أكثر الأصحاب هو أن المراد به من نصب العداوة لآل بيت محمد (ص)
"What is reported is that he is impure [najis] and that he is worse than the Jew, Christian and the Magian; and he is an impure kafir by the unanimity of the scholars of Shi`a Imamiyya ridwanullahi `alayhim; and what the majority of scholars have opined is that what is meant by nasb [i.e., the trait that makes one Nasibi] is enmity for the progeny of Muhammad sallallahu `alayhi wasallam."
Ayatollah al-Sayyid Murtada al-`Askari says in Ma`alim al-Madrasatayn [1:81] that "Nasibi" according to them is:
كل من يبغض الأئمة من أهل البيت عليهم السلام
"Everyone who hates the Imams of Ahl al-Bayt."
Grand Ayatollah al-Sayyid Bashir al-Najafi describes him in one of his answers as:
الناصبي هو الذي يظهر العداء لأحد المعصومين ابتداءً من الرسول الأعظم صلى الله عليه وآله وسلم ، إلى الإمام المنتظر والزهراء عليهم السلام وأما السب فاجتنبه ، ويجوز لك أن تلعنه بينك وبين ربك ، وتلعن من لا يلعنه ، ولكن فليكن ذلك بينك وبين الله لتحظى بالأجر على البراءة من أعداء الله ورسوله وأعداء ذريته لعنهم الله
"Nasibi is he who makes (his) enmity for anyone of the infallibles obvious, starting from the greatest Messenger sallalahu `alayhi wasallam to the awaited Imam and al-Zahra' `alayhim al-salam. As for insulting him, then take precaution; and it is permissible for you that you curse him between you and your Lord, and that you curse whoever doesn’t curse him, but that will be between you and Allah because you are receiving reward over disassociation from the enemies of Allah, His Messenger and the enemies of his projeny la`anahum allah."
So according to Ithna' `Ashari scholars, a Nasibi is one who hates the progeny of Muhammad (as), and he is kafir, impure [najis], worse than Jews, Christians and Magians; deserves curse [la`na] and resembles dogs and pigs, and his blood should be spilled.
[II] Sunni `Ulama Labeled "Nasibi" by Ithna' `Ashari Scholars
The following is a list of some of our `ulama that have been labeled as Nasibis by Ithna' `Ashari scholars. I will list them one by one and then cite/quote Ithna' `Ashari scholars as evidence.
 Imam al-Sha`bi
Grand Ayatollah Hasan al-Lawasani mentions him in Nur al-Afham [2:34] as:
وعامر الشعبي الناصبي المنحرف عن أمير المؤمنين ( عليه السلام
"...and `Amir al-Sha`bi, the Nasibi, perverted about Amir al-Mu'minin..."
 Imam Malik
Muhammad al-Tijani says in The Shi`a are Real Ahl al-Sunna (Arabic ):
ومما سبق نفهم بأن الإمام مالكا كان من النواصب
"From the above text we can understand that Imam Malik was a Nasibi..."
 Imam al-Shafi`i
al-Muhaqqiq al-Bahrani has written poetry against Imam al-Shafi`i. He starts it with:
كذبت في دعواك يا شافعي * فلعنة الله على الكاذب
"You lied in your claim, O Shafi`i! and the curse of Allah is upon the liar."
After a couple of lines, he says:
فالشرع والتوحيد في معزل * عن معشر النصاب يا ناصبي
"Shari`a and tawhid is in a different place, from the group of Nasibis, O Nasibi!"
 Imam Ahmad
al-Kash-shi says about him -- per al-Arba`in  by al-Shirazi:
هو من أولاد ذي الثدية ، جاهل ، شديد النصب
"He is from the offsprings of Dhul Thudayya; an ignorant, extreme in nasb..."
 Imam Ibn Ma`in
Grand Ayatollah al-Mar`ashi says in Sharh Ihqaq al-Haq [7:398]:
وكذا الكلام في يحيى بن معين ، فإنه كان أمويا ناصبيا
"The same speech goes for Yahya ibn Ma`in, for he was an Umayyad Nasibi..."
 Imam Abu Dawud
Shaykh Baqir al-Mahmudi refers to him in the hashiya of Jawahir al-Matalib  as:
وهو عبد الله بن سليمان بن الأشعث الناصبي
"He is `Abdullah ibn Sulayman ibn al-Ash`at the Nasibi..."
 Imam Ibn Hibban
Ayatollah Taqi al-Tustari refers to his opinion in Qamus al-Rijal [9:283] as:
قول ابن حبان الناصبي
"...The saying of Ibn Hibban the Nasibi..."
Grand Ayatollah Muhsin al-Amin mentions it as:
قول ابن حبان الناصبي المعروف
"...Saying of the known Nasibi Ibn Hibban..."
 Imam al-Daraqutni
Ayatollah `Ali al-Namazi says about him in Mustadrakat [8:514]:
"He was a Nasibi."
 Imam Abu Bakr al-Baqillani
Shaykh Muhsin al-Mu`allim counts him in his list of Nasibis in his book al-Nasb wa al-Nawasib , and then mentions reasons why he is supposedly a Nasibi.
 Abu Hayyan al-Tawhidi
Ayatollah al-Sayyid Murtada al-`Amili says in al-Sahih min Sirat al-Nabi [5:278]
ومن الأمور الطريفة هنا : أن أبا حيان التوحيدي - الناصبي المعروف - يروي
"Among the strange points here is that Abu Hayyan al-Tawhidi, the known Nasibi, narrates..."
 al-Khatib al-Baghdadi
Taqi al-Tustari says in the Qamus al-Rijal [9:390]:
ولا عبرة بقول الخطيب الناصبي
"... And no importance is to be given to the opinion of the Nasibi al-Khatib."
And quotes him as [9:555]:
وفي تاريخ بغداد للخطيب الناصبي
"It is in Tarikh Baghdad by the Nasibi al-Khatib..."
 Imam al-Ghazali
al-Muqaddas al-Ardabili says -- per Tara'if al-Maqal [1:122] -- while mentioning the Imam's alleged discussion with al-Sayyid al-Murtada ibn al-Da`i al-Hasani:
السيد المرتضى الذي جرت له المباحثة مع الغزالي الناصبي هو ابن الداعي الحسني
"The Sayyid Murtada, between whom and the Nasibi al-Ghazali the conversation took place, is Ibn al-Da`i al-Hasani."
Sulayman al-Mahuzi says in al-Arba`in  when referring to al-Ghazali:
والعجب من هذا الناصب كيف
"...And it is astonishing how this Nasibi..."
 Imam Ibn al-Jawzi
Ayatollah Taqi al-Tustari mentions him in Qamus al-Rijal [11:599] as:
ابن الجوزي هو " عبد الرحمن بن علي الناصبي "
"Ibn al-Jawzi: He is `Abdul Rahman ibn `Ali, the Nasibi..."
 Imam al-Fakhr al-Razi
Muhammad Baqir al-Majlisi mentions Imam al-Razi as a Nasibi in several of his works. In Bihar al-Anwar [35:384], he quotes him as:
وأفحش من ذلك ما ذكره الرازي الناصبي حيث قال
"And more indecent than that is what was mentioned by the Nasibi al-Razi when he said..."
Sulayman al-Mahuzi says in al-Arba`in  after quoting him:
العجب من قول هذا الناصب كيف
"The amazing thing from the saying of this Nasibi is that how..."
Ni`matullah al-Jaza'iri also refers to him in Nur al-Barahin [2:228] as:
حتى أن الناصبي فخر الرازي ذكر في خاتمة كتاب المحصل
"...to the extent that the Nasibi Fakhr al-Razi mentioned in the end of the book al-Muhassal..."
After this, al-Jaza'iri quotes another thing and says:
وهذا الكلام صريح في كفر هذين الرجلين الرازي وابن جرير
"This expression is explicit in the kufr of these two men, al-Razi and (Sulayman) ibn Jarir..."
 Imam al-Dhahabi
al-Shahid al-Thalith Nurullah al-Shustri in his refutation of Ibn Hajar al-Haythami, refers to al-Dhahabi in al-Sawarim al-Muhriqa  as:
وأما ما نقله عن الذهبي الناصبي ذهب الله بنوره
"As for what he has copied from the Nasibi al-Dhahabi, may Allah take the nur away from him..."
Muhsin al-Amin says in A`yan al-Shi`a [3:294]:
ذكره الذهبي الناصبي في ميزانه
"He was mentioned by the Nasibi al-Dhahabi in his Mizan..."
And Ayatollah `Ali al-Korani quotes him in Ma`rifat Allah as:
قال الذهبي الناصبي في ميزان الإعتدال
"The Nasibi al-Dhahabi said in Mizan al-I`tidal..."
 Ibn Kathir
Ayatollah Muhiyy al-Din al-Mamaqani says in the hashiya of Tanqih al-Maqal [5:175]:
وفي البداية والنهاية للناصبي ابن كثير
"It is in al-Bidaya wa al-Nihaya by the Nasibi Ibn Kathir..."
Shaykh Baqir al-Mahmudi says in his hashiya of Shawahid al-Tanzil [2:192]
كذا رواه الناصبي المحترف ابن كثير
"And it was reported similarly by the professional Nasibi Ibn Kathir..."
Shaykh Muhsin al-Mu`allim also labels him as a Nasibi in al-Nasb wa al-Nawasib 
al-Mahuzi says in al-Arba`in  about the author of al-Mawaqif, al-Iyji:
اعترض القاضي الناصب في المواقف
"The Nasibi Qadi objected in al-Mawaqif..."
He later says in the same book :
أن القاضي المتعصب الناصب في المواقف أورد هذا الخبر
"...That the bigot Nasibi Qadi documented this report in al-Mawaqif..."
 Shaykh al-Islam Ibn Hajar al-`Asqalani
Ayatollah Taqi al-Tustari says about a narrator in Qamus al-Rijal [9:286]:
وثقه ابن حجر الناصبي
"...He was declared trustworthy by the Nasibi Ibn Hajar..."
 Hafiz al-Suyuti
Sulayman al-Majuzi says in al-Arba`in , about al-Suyuti's interpretation:
وعلى كل حال فتأويل هذا الناصب الجاهل
"In any case, the interpretation of this ignorant Nasibi..."
 Shaykh Ibn Hajar al-Haythami
Ni`matullah al-Jaza'iri says in Nur al-Barahin [2:158]:
ولما نظر ابن حجر الناصبي إلى أن هذا الخبر يستلزم
"When Ibn Hajar the Nasibi viewed that this report necessitates..."
al-Qadi Nurullah al-Shustari mentions him in al-Sawarim al-Muhriqa  as:
هذا الشيخ المتعصب الجامد الناصبي
"This prejudiced, arrogant and Nasibi Shaykh..."
al-Sayyid Hasan Aal al-Mujaddid al-Shirazi says in al-Nisal al-Khariqa -- per Turathuna [50/239] -- after quoting Ibn Hajar:
فهل يستجيز هذا الناصب أن يقول في كتابه
"So does this Nasibi ask for permission when he says in his book..."
The reason why Ibn Hajar got fame as a "Nasibi" among them is his refutation of them known as al-Sawa`iq al-Muhriqa, even though he filled his book with countless virtues of Ahl al-Bayt which are oft-quoted by Ithna' `Asharis. Muhsin al-Mu`allim counts him as a Nasibi in al-Nasb wa al-Nawasib  and Mirza `Ali Muhammad Khan even named his refutation of Ibn Hajar's al-Sawa`iq: "al-Shuhub al-Thawaqib fi Tard al-Shaytan al-Nasib" ("The Shooting Stars in Repelling the Nasibi Devil")!
 Shah Waliullah al-Dehlawi
Ayatollah `Ali al-Milani says in Nafahat al-Azhar [19:415] while mentioning Waliullah's son `Abdul `Aziz:
قد شبه ( الدهلوي ) انشعاب السلاسل من أمير المؤمنين عليه السلام إلى الشعب المختلفة...خلافا لوالده صاحب قرة العينين ، وغيره من النواصب
"al-Dehlawi likened the branching of the silsilas of Amir al-Mu'minin `alayhis salam to to different branches... as opposed to his father, the author of Qurrat al-`Aynayn, and other Nasibis..."
 Shah `Abdul `Aziz al-Dehlawi
Mirza `Abdullah Efendi al-Isfahani refers to him in Riyad al-`Ulama as -- per Khatimat al-Mustadrak [2:175] by al-Nuri:
عبد العزيز الناصبي الدهلوي ذكر في التحفة
"...The Nasibi`Abdul `Aziz mentioned in the Tuhfa..."
Ayatollah `Ali al-Milani mentions in Nafahat al-Azhar [4:271] that some of the top Ithna' `Ashari scholars said in reply to Shah `Abdul `Aziz:
أما ما ذكره هذا الناصبي عن النواصب
"As for what was mentioned about the Nasibis by this Nasibi..."
 Mahmud Shukri al-Alusi
Shaykh al-Sayyid Baqir al-Hujja wrote a refutation of his attacks on Ithna' `Ashari creed and named it: "al-Sahm al-Thaqib fi Radd Ma Laffaqahu al-Nasib" ("The Shooting Arrow in Refutation of What Was Fabricated by the Nasibi")
This is what I found in a quick research; I'm sure that if one attempts to futher research the topic thoroughly then a lot more can be found, in fact, a whole book can be compiled showing how our scholars have been labeled as "Nasibis" by Ithna' `Ashari scholars. For now, I think the above should be enough in showing the "other" side of the picture for those who promote Shi`a-Sunni unity.