My favorites in order
#1 Unisystem classic and cinematic (Buffy/Angel, All Flesh Must be Eaten witchcraft more) . A wonderful game that is a bit moribund right now. Not dead just resting waiting for BRAAAINS...
#2 GURPS. If I could choose only one system, I'd choose Big G no hesitation. It might be the best RPG ever designed
#3 Beyond the Wall. A D&D variant that oh heck I'll link its main page and let them describe it.
#4 Rolemaster. I don't play this old warhorse these days but I've never had a bad game.
#5 Every other D&D variant. I like 2e, 3x and 5 mostly though Swords and Wizardry is a solid choice too.
The Return of Planescape? - Curious. There is a brief note on this 'countdown' at Enworld.
15 hours ago