I did own a Genesis, but I never got into it like I did the SNES. My brother probably played it more than I did. Anyway, out of the extremely limited amount of games I played for it, I’d have to say Sonic 2 was my favorite. I know its a pretty common answer, but the only games I can actually remember owning were Sonic, Sonic 2, and Toe Jam and Earl 2. I know there were 2-3 others, I just can’t remember them.
And by the way, your comment about Donald Duck and plungers makes me think how much better Kingdom Hearts would have been if Donald had cast magic with a plunger rather than a staff.