Sonic the Hedgehog 1-2-3, obviously.
Indeed, I'd also list Sonic CD, Sonic & Knuckles and Sonic 3D Blast. Then that list would sum up my childhood nicely.
You mention the Genesis without saying Phantasy Star II-IV, for shame!
I grew up with a Genesis, but I haven't played any of the Phantasy Star titles beyond the original.
Some of my other favourites would include Contra: Hard Corps, Castlevania: Bloodlines, Teenage Mutant Ninja Turtles IV: Turtles in Time, Streets of Rage 2 and Gunstar Heroes.