Sega Genesis for me because of Sonic, Streets of Rage, and sports games. I'd take the entire collection of Sonic over all the platformers the SNES has to offer, and I've logged hundreds of hours into Streets of Rage 2, constantly trying to get better at it (I've yet to beat the game starting with 1 life, no continues, on Mania difficulty). Sonic 3 & Knuckles is my favorite platformer of all time.
It's odd though, considering the SNES is home to many games that I love.
Currently playing: Uncharted: Drake's Fortune, NBA2k11, Metal Gear Solid, Picross 3d