What are some good side scrollers?

the Metroid games are good too, at least from what I've seen. I've never actually played any of them all the way through. And there's always mario I've heard good things about the new TMNT too (GBA version, I hear all the other versions suck).

 Edit: TMNT is more of a beat-em-up than a side-scroller.

Aren't we forgetting about MegaMan?

I have Megaman & Bass; it's a challenging game with plenty of variation. And James Pond: Robocod  is pretty legit.