Mana Vault Mana Vault is part of a nasty 2-card combo: Mana Vault plus whatever busted 4- or 5-mana spell you want to play on turn 2 strong enough to be banned in Legacy and even restricted in Vintage
You can add one to: Choose up to one target artifact, up to one target creature, and up to one target land
Check out the full precon decklists.
VAT OF REBIRTH This one is such a no-brainer in any deck that uses the graveyard judiciously
Serum Visions, Opt, Sleight of Hand and Cling to Dust have all made the deck at some point, but none of the Modern cantrips are quite strong enough to match the consistency of something like Ponder
Though it has had many analogous cards printed, it remains arguably the best, because its two mana, and it can bring an untapped land