How old school? If you mean early 80s, it would be Miner 49er on the Atari 400. If you mean mid 80s, then Castlevania II: Simon's Quest, even though that game was obviously incomplete, but still a nice adventure. If would be Battle of Olympus if that game didn't have 3 million digit long codes or weapons and items that costed too dang much! Of course, if there was someone else over to play with me with a Turbo Controller, then I would have to pick Double Dragon II, which was really fun as long as you had a turbo controller and were not alone. Otherwise, it sucked!. If you mean the late 80s (Hammer Time!), then Castlevania 3: Dracula's Curse. If you mean the 90s, then Streat Fighter II: Hyper.

Once you reach 1999, they just weren't old school anymore to me.