On a Sufi website, I read that Ibn Taymiyyah, Ath-Thahabi and Ibn Al-Qayyim were Sufis. Is that correct? Why was Ibn Taymiyyah buried among Sufi graves? Are Sufis considered to be a group of Ahl As-Sunnah, or are they a deviant group?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.

Ibn Taymiyyah explained the practices and  customs of Sufism in multiple Fatwas. He gathered all these Fatwas in one volume entitled Kitaab At-Tasawwuf.

Although Ibn Taymiyyah praised some Sufis, he was not one of them. He believed that the Waliyys (allies of Allaah The Almighty) are pious Muslims, whether they are the poor, Sufis, scholars of Fiqh (Islamic jurisprudence), scholars, traders, workers, emirs or governors. Allaah The Almighty Says (what means):  {Unquestionably, [for] the allies of Allaah there will be no fear concerning them, nor will they grieve. Those who believed and were fearing Allaah.} [Quran 10:62]

Ibn Taymiyyah was not a Sufi even though he praised some Sufis. He also criticized those who claimed to be Sufis, for example, Al-Hallaaj and Ibn ‘Arabi. He declared some Sufis as people of Bid‘ah (religious innovators) and Zindeeqs (persons professing Islam and hiding disbelief). Scholars who are competent researchers did not consider some people like Al-Hallaaj, to be Sufis.

We do not know why Ibn Taymiyyah was buried in a graveyard belonging to the Sufis. He was buried near his brother, Sharaf Ad-Deen ‘Abd Allaah . Burying Muslims in Sufi graves is not impermissible because the Sufis are still Muslims. In principle, Muslims should be buried in the graveyards of the Muslims. We know that the graveyard where Ibn Taymiyyah was buried did not exclusively belong to Sufis; rather other non-Sufi Muslims were buried there.

Allaah Knows best.