Chromium, the Mutable

Oracle Text
Flash
This spell can't be countered.
Flying
Discard a card: Until end of turn, Chromium, the Mutable becomes a Human with base power and toughness 1/1, loses all abilities, and gains hexproof. It can't be blocked this turn.
For example, you flash him in during your opponent's end phase, your opponent can't counterspell him. Your opponent needs other instant speed answers to respond, but because of Chromium's built-in protection, it may not be worth it if you have cards in hand. Then during your turn, your lands untap so you can equip and/or enchant Chromium and swing since Chromium isn't affected by summoning sickness.
Since Chromium becomes unblockable in his human form, you can also choose for a strategy that uses Quietus Spike and Scytheclaw to whittle away your opponent's health without going for just 3 hit KO with just commander damage.
Basically one of the more straight forward and flexible control legendarys.