Here are some alternatives of whhich creatures i think is on top of the food chain in the forgotten realms. Do you have any specific characters or creatures to add?
Creatures from any ruleset 2.0 to 3.5 are welcome.
Balor's Solar's Tarrasque Titan's Leviathan Dragon's Pit Fiend's Liche's Beholder's - added by Sylvus_MoonbowSuccubus Succubus - added by Zeratul (cuz their hot...)
Elminster Gromph - Archmage of the underdark Demogorgon - Prince of Demons - added by doppleganger95 Larloch - Shadow King - Added by Lapa Khelben "Blackstaff" Arunsun - Lord Mage of Waterdeep - added by Aosaw