Has any of the Sunni scholars confessed to the authenticity of the hadith of Qadir (Imam Ali’s succession to the Prophet)?
1) Tirmidhi D. 279 A.H.
He owns one of the six Sunni “Sahih” books, and right after the narration of this Hadith says:
هذا حَدِيثٌ حَسَنٌ صحيح.
It is an all right and authentic Hadith. 
Mr. Abu Ja’far Tahawi who is referred to as an Imam and his books are among the curriculum of all Islamic sects even Wahhabism, after narrating this Hadith, says:
This Hadith is a “Sahih” (Authentic) one and there is nothing wrong with the narration of Qadir Hadith. 
3) Ibn Abd-al-Barr Qurtubi D. 463 A.H.
He, after narrating the Qadir Hadith, says:
وهذه كلها آثار ثابتة.
All the Qadir consequences and Hadiths are fixed and proved as authentic. 
4) Al-Hakim Nishapuri
He mentions numerous ways and says: “This narration is correct.”
5) Ismail ibn Kathir:
Ibn Kathir says:
قال شيخنا أبو عبد الله الذهبي وهذا حديث صحيح.
Our master Abu Abdullah al-Dhahabi has said: “This is a correct Hadith.” 
Ibn Kathir then says:
قال شيخنا ابو عبد الله الذهبي الحديث متواتر أتيقن أن رسول الله قاله.
Our master Abu Abdullah al-Dhahabi has said: “This is a frequently narrated Hadith and I have no doubt that it has been narrated by Prophet Muhammad (PBUH) himself.” 
7) Ibn Hajar al-Asqalani:
كثير من اسانيدها صحاح وحسان.
Many of its (Qadir Hadith) documents and references are “all right” and “Correct”. 
8) Zia-al-din Maqbali D. 1108 A.H.
He is one of the greatest Sunni scholars and he is highly respected and praised a lot. He says:
فإن لم يكن هذا معلوما فما في الدين معلوم.
If the hadith of Qadir is not definite and clear, we will not have any definite and clear word in the religion (Islam). 
If a narration which is narrated by 130 of the Companions (Sahabi) through numerous ways, and even some have mentioned 250 documents and references for it, can be uestioned and doubted then we won’t have any definite narration in Islam to refer to and depend on for further action.
9) Mirza Muhammad Badakhshi
وهذا حديث صحيح مشهور، ولم يتكلم في صحته إلا متعصب جاحد.
“No one questions the authenticity of Qadir Hadith unless he is a dogmatic person and this dogmatism prevents him from realizing the truth or is willfully ignorant which means that although he is sure of its authenticity, he denies it.” 
He then says:
لا اعتبار بقوله
“This person’s words are not worthy.”
10) Mahmud al-Alusi
نعم ثبت عندنا أنه (صلي الله تعالي عليه وسلم) قال في حق الأمير كرم الله تعالي وجهه هناك: من كنت مولاه فعلي مولاه.
“Yes, it has been proven to us that in Qadir Prophet Muhammad (PBUH), about Imam Ali (PBUH), had said: “Whoever I have been his master, from now on Ali is his master.” 
11) Abu Hamid Ghazali
He is referred to as Imam Ghazali. He says:
وأجمع الجماهير علي متن الحديث من خطبته في يوم غدير خم باتفاق الجميع.
“Almost all Sunnis agree on the text of the Qadir hadith.” 
12) Abu al-Makarim Ala- aldin Semnani
هذا حديث متفق علي صحته.
“This is a hadith with which everyone agrees on its authenticity unanimously.”
13) Ibn al-Jazari D. 833 A.H.
He says: “This is a frequently narrated narration and it is narrated by many and those who want to degrade it are uneducated and don’t know Elm al-Hadith (Hadith studies).” 
ممن لا اطلاع له في هذا العلم.
Manawi quotes Suyuti as saying the hadith is frequently narrated). 
15) Mulla Ali al-Qari:
This is a frequently narrated hadith and there is no doubt in that and 30 of the Companions (Sahabi) had testified to its authenticity when some were quarreling over Ali’s caliphate.
16) Al-Ajlouni D. 962 A.D.:
فالحديث متواتر أو مشهور.
This hadith is either frequently narrated or famous. 
He is referred to as the contemporary “Bukhari”:
حديث الترجمة حديث صحيح بشطريه، بل الأول منه متواتر عنه صلي الله عليه وسلم كما ظهر لمن تتبع أسانيده و طرقه.
The Hadith of Qadir is translated correctly and two parts of it “Whomever I am his master, thus Ali is his, too” and the next part “May Our Lord love whomever, loves Ali and may take those who consider Ali as their enemy as His enemy.” are all right. The first part is frequently narrated and whomever follows and reasons the Hadith, will find it reasonable and will conclude the outcome (the second part).
18) Shoaib Arna’oot (Birth 1928)
Mr. Shoaib Arna’oot who is one the famous Sunni scholars, says:
There is a lot of evidence for the Qadir Hadith that reach to the level of frequency.
Late Allameh Amini and Honorable Ayatollah Milani in Nafhat al-Azhar have quoted 60 and 63 of the Sunni scholars who either have testified to the authenticity of Qadir Hadith or have confessed that it is a frequently narrated Hadith.
 Jami’ at-Tirmidhi, V5, P 633 Ibn Taymiyyah in Minhaj as-Sunnah an-Nabawiyyah V7, P 320 betrays and says : “Hasanah Tirmidhi”
 Moshkel Al-Asaar. V2, P308
 Al-Isteeaab. V3, P1100
 Al-Bedayeh va Al-Nahayah. V5, P209
 Al-Bedayeh va Al-Nahayah. V5, P214
 Fath Al-Bari. V7, P74
 Shawkani says about Zia-al-din Maqbali : “وهو ممن برع في جميع علوم الكتاب والسنة… وفاق في جميع ذلك وله مؤلفات مقبوله كلها عند العلماء.” (Al-Badr Al-Taale V1, P 288)
Mr. Ziauddin Muqbil is a person who, in Quranic and Sunnah sciences, is among pioneers and has reached the ultimate level of perfection and all his publications are highly accepted among other scholars.
 Abhas Masdadah. P122
 Nuzul al-abrār bi-mā ṣaḥḥa min manāqib Ahl al-Bayt al-Aṭhār. P21
 Ruh al-Ma’ani. V6, P61
 Ser Al-Alamin. P9
 Al-Qadir, Al-Sheikh Al- Amini. V1, P299
 Feiz Al-Qadir. V6, P218
 Kashf Al-Khafaa. V2, P361