AD&D is fun, but challenging (especially on the two hardest difficulties). Atlantis is brutally hard but fun. Now for stuff you don't have? Anything by AtariSoft is excellent, Dig Dug and Commando were fun (if slightly pricier-to-get) ports. If you enjoy roguelikes, nothing beats Tower of Doom on the INTV. Thunder Castle's a great game with very nice graphics and surprisingly-good music. Burgertime is a common but amazingly well done port of the arcade game. Activision made some good titles (Pitfall, Happy Trails, Stampede, Beamrider), TRON: Deadly Discs was fun.
I'll think of more over time.