1. Ristar
You may be wondering how Sonic got the number 2 spot. This is why. This game was also made by the Sonic Team, but to me, it's my favorite game of all time off the Sega Genesis. You play as a star, who is attempting to overthrow an evil ruler and rescue his father. You attack by headbutting things, and the game is stylish as the day is long. Every last track is memorable, the levels are diverse, the bosses fun and challenging, and the final boss fight is intense. This is the Genesis game I'd recommend more than any other. Play it. Just do it.
http://www.youtube.com/watch?v=80XLkViR3LY