All things Dragon Warrior/Quest. The Monster ones were better than the Warrior ones, but the Warrior ones were definitely more complex story-line wise. I've played:
Dragon Warrior Monsters I, II, III, and Joker. DWM I is the one game I definitely put the most time in to.
Dragon Quest/Warrior I, II, maybe III, IV, V, and I just "finished" IX.
I'm looking forward to DWM Joker II, and I wish that I knew Japanese so I could play it already. The game came out in Japan a long time ago and I don't think they've even announced a launch date for an English version, although there definitely will be one.