The first Rayman, played on the Sega Saturn. Probably the best 2D platformer ever in my opinion, and a shame Ubisoft never matched the quality of that game ever again (never played Rayman 2 and 3 and all those Rabbids spinoffs, but looking at them through YouTube they aren't particularly amazing and colorful compared to the first title). I really need to repair both my Saturn and the disc so I can play this masterpiece again.
Runner ups... probably Sonic 3 & Knuckles (Megadrive/Genesis), Super Mario Bros. (NES) and Okami (PS2 version so far).