Of all the classic games in Sega Genesis Collection, Altered Beast is probably the one I have the most fond memories of. I remember, as a wee lad, going over my cousin's house to see him play it on his new Sega Genesis. Back then, the graphics were so unbelievably awesome, and when I saw the main character transform into a werewolf, I just about died with excitement. It's truly a classic.
And the Vectorman series is a classic in its own right. In Vectorman 2, which was released in 1996, you are charged with battlng mutant insects and ultimately saving the world by defeating the Black Widow Queen.