{Weird Harvest} with {Aven Mindcensor} in play. Works best if you've got an infinite mana combo like {Lotus Cobra}, {Grazing Gladehart}, {Fastbond} and 2 Ravnica "return a land" lands ({Simic Growth Chamber}). Also infinite life if you use those cards.
You can also just return the land you keep playing, now that I pay attention to the wording of the Ravnica guild lands.