I'm making my way through FF7 again, have started Fire Emblem and Golden Sun and loving both. I'm also rage quitting my way through the various R-Type games (mostly SNES), hot damn those are hard, good fun though.
My favourite on its own would be Sonic and Knuckles, but the best, imho, is to combine Sonic 3 with Sonic & Knuckles. You used to be able to actually plug one cartridge into the other - so cool
Oh right I remember that, the Knuckles cartridge had those little flaps at the top you could jam another cartridge into. My young mind was blown when I saw that for the first time.
I have an external soft spot for Sonic 2, but objectively S&K is probably the best of the bunch.