Shadow of the Colossus for me, and it's not even remotely close.
Super Mario Brothers is obviously one of the greatest franchises in gaming history. But the original just doesn't stack up to some of the sequels, and is rather weak all these years later if I may say so myself. There are already plenty of Mario games advancing in this tournament, we don't need yet another one.
Colossus is another one of those games like Ico that transcends video gaming and turns itself into a work of art. Sure there's not really much of a story to be found, but it's not needed. Absolutely beautiful game in every way, some of the absolute greatest and most massive bosses in the history of gaming, and completely unique in every way. Stunning is the only word one can use to describe this game.
Don't be a tool, vote for the superior piece of ART here, and that is Shadow of the Colossus in every way.