https://classic.wowhead.com/guides/classic-world-of-warcraft is the best source I’ve found for up-to-date information on the specifics of Classic vs. Vanilla.
So some preraid BiS will be from Maraudon and Blackrock Depths, such as these:
https://classic.wowhead.com/item=17719/inventors-focal-sword
https://classic.wowhead.com/item=17718/gizlocks-hypertech-buckler
https://classic.wowhead.com/item=11662/banthok-sash