I haven't seen Turn A yet but out of the series I have seen I would say it's either Devil Gundam (ability to destroy and recreate whole planets, evolve and self heal).
That or the 00 Raiser in Trans AM Burst mode simply because it is almost indestructable at that phase and the giant mass of GN particles it releases gives the pilot the ability to influence the decision making of everyone in the area(maybe it's just Setsuna but either way). That combined with the Giant beam saber capable of wiping out space stations and entire fleets make him almost, if not, unbeatable.
:High School Of The Dead Signature by Maszrum:
Name: Nakushita Chi(Lost Power/Or Energy)
Released Form: The Zan splits into two twin zans connected by a chain at the bottom side of the hilts. The length of the chain initialy is 4 feet in length but can shorten or lengthen based on the style of attack and how much reiatsu is channeled into the chain.
Abilities:Nullification: The blades nullify all elemental and kido based attacks as long as one of the blades comes in contact with the attack.
:Tobi Saru(fly away):While holding onto both zans all reiatsu is channeled into the chain and the zan on the right, allowing it to be thrown at high speed toward the oponent. The path of the released zan can be controlled by placing both hands on the remaining twin and thus allowing the imediate return of the twin after the initial attack.
Turn A Gundam with the ability to turn every thing back to stone age.
Gundam 00 Raiser, one of the reasons is because of it's Trans-Am Raiser system giving him the ability to transport in actual combat and also the powerful Raiser Sword. The almighty mode called the Trans-Am Burst mode can make this baby unstoppable. It is a complete and powerful MS in short, mid and long range combat.