I'll start off with a pretty common one, and that's Metal Slug. A lot of people I know would say Metal Slug 3 instead, and granted that's a great game, the original was just incredible the first time I played it. I'm still as into it as I ever was.
Windjammers. Definitely the best extreme frisbee game I've ever played, haha. The single player game is pretty average, but throw a couple of friends into the mix and there's more trash talking and yelling than I've ever heard, and it rules.
Blazing Star is one of my favorite shooters ever. It's just so ridiculous and over the top I can't help but love it. IT'S LIKE A RAINBOW SPACESHIP EXPLOSION