Starcraft II Coop: The game inside the game.

[The_Snark]

Yeah, this. Tychus has the heightest floor of the commanders, even if you take the worst of his mercs he is pretty decent.

I believe the heighest ceiling are kerrigan and Vorazun based on guides. Watching a good Kerrigan player makes me feel like a scrub.

I think Tychus also benefits a lot from how the economy works in co-op - most people end up with tons of minerals (and sometimes vespene) going spare, because they're at their supply cap and can only research upgrades so fast (or they run out). Tychus, on the other hand, has a ton of very expensive instant upgrades.

I've no idea who the best is, but Kerrigan is definitely high-variance. The hero unit can do ridiculous things if used right, but if you mess up she dies really fast.