Without a doubt, I would have to choose the original Vectorman. When it was released in 1995, I couldn't wait to play it. It had everything you could want in a game: great stylized graphics, sound, and most importantly, gameplay. You could morph into different things, including a tank, airplane, and even a bomb (in case you're in the mood to blow up your enemies along with yourself).
On top of all this, Sega announced shortly thereafter that only a few cartridges had a special bonus feature: after beating the game, you could get a "You Won!" screen with a special phone number and code that won you all sorts of great prizes and money. If that didn't tempt you into playing the game, nothing would.
And one other thing, which I thought was both ingenious and annoying: if you cheated to beat the game (which of course I did), you got another message: "Please play through the game again without cheating." You have to laugh at that. :thumbup: