Zone or 99: Both Gandalf the Grey While it appears that his abilities run out, keep in mind that all you have to do to reset him is make him leave the battlefield
With the menacing Giovanni in the foreground, and the awesome Mewtwo softly pulsing with Psychic energy in the background, looming over you, it has quickly become the most valuable modern Mewtwo ever printed
I just wanted to give a little background as to why the format looks like it does
For Winter Orb, this will be artifact and creature sources of mana that continue to untap regularly, or a way to tap the Winter Orb so that it's not untapped when your turn rolls around, and you aren't effected by it, like Urza, Lord High Artificer or Derevi, Empyrial Tactician or the new Galazeth Prismari (there's no rule that says you can't generate mana you can't spend, and if you think this is a hint about a forthcoming article, then, you're right) A Stax deck will also need a win condition, since it's not enough to merely grind the board to a halt