Well, depends on how old-school you mean, but since my favorite game ever is Star Ocean: The Second Story, and that's fairly dated by now (it's an original PSX game), then that. Unless by old-school you mean like NES or older, in which case it'd probably be... I dunno... the Dragon Warrior games?
And on a related note... am I the only person in existence who actually *enjoyed* the E.T. game for the Atari 2600? Of course, I was pretty damn young at the time, but still. :/