Starcraft - That game is so well made that it's still really fun to play today.
Halo 3 - I'm a Halo junkie. I enjoyed the adventures of Master Chief and I like playing it online, even though I'm not very good.
Archon - One of my favorite games on the Commodore 64. I had some awesome times playing that.
Mail Order Monsters - Similar battle mechanism as Archon, but you could customize your characters. What's not to like?
Street Fighter II - I could elaborate. Or I could just kick your butt. Sonic Boom!