WaffenSS Killer: The instant reinforcements (ODST Drop) and the orbital PWN cannon allows Cutter to stop any gaps in his line rather swiftly. Carpet Bomb and Cryo just don't get the job done like the MAC.
Cov sucks at turtles due to the fact that their anti-vehicle SUCKS compared to the Cobra when placed on defense.
EDIT: Though I could make a strong case for the Arbiter.
covie sucks cause they need to have 2 slots on there base to get vampires , jackals and hunters(factory , hall)
plus another slot for a sheild generator , and another for a temple , which leaves 3 supply pads ,
which is why i think anders is the best cause her upgrades cost half price so she can deal with only 3 supply pads