Lots of great suggestions here, but if you've never played the original Super Mario Bros, Super Mario Bros 3, and Super Mario World, you should. There is a reason they're considered classics and everybody should experience Mario's roots. My 8 year old plays them every so often and he keeps progressing.

SMB 2 is a good game as well, but it's quite different from the others. It is the origin of a few series staples, though.