RPGs , while I love them, would get boring really fast if they were all I have to play. Xenogears is my hands down all time favorite game ever, but I'd probably not pick that, it's not really that replayable.
I'd rather choose a game I can always play over and over again and have fun with, or a game in which I can always learn something new by toying around with it (i.e. games that are more toys than games). Probably either Civilization 5, Ultimate Marvel vs. Capcom 3 or a Beatmania IIDX game.