[Amhazair]

There are quite a few very good artifacts, from kickass weapons like the Mage Bane, to the ridiculous gem savings from the Hammer of the Forge lord, to excellent boosters, to the awesomeness that is the Chalice.

But there really is nothing that comes even remotely close to the game-changing potential of the Gate Stone and the Boots of the Planes. Those two items just give you an unparallelled flexibility that can't be duplicated by anything else in the game, and I would gladly trade my soul for either of them.