Which AI in Skirmish is the toughest to beat?

In your own opinion? Mine is Guerilla... the AI KNOWS how to counter you/

(P.S: By the way, what's the difference between Rusher and Steamroller? I played against both AI and they just act the same...?)

You mean AI personality? Yup,hardest is Guerilla. They know your weak points and all.

The difference in Rusher and Steamroller is WAY different. Steamrollers attack you at a late point in the game but they spam Tier 3 units and send them ALL to your base.

Rusher is a bit like the opposite. They attack early in the game with small forces. They either cripple your base,cripple your economy or utterly defeat you from the game.

that depends on your playing style. I find that steamroller is toughest, since all you have to do to beat a guerrilla is hold back the few initial attacks while you set up proper defenses for the larger attacks to come. although I may change my mind once the patch significantly slows the economy.

but currently, I find the steamroller's massive attacks more of a problem, at least with brutal AI (I find that hard and down pose no challenge at all under any personality).
although guerilla is still more of a threat then rusher, as all you have to do is hold off the initial 2-3 attacks, or turtle, which is plain broken, and the only brutal I can beat 1v1 on a regular basis. balanced is also not much more then a rusher which techs up a bit faster, and invests in more aircraft then steamroller.

Steamroller is just ughhhhhhhh. I definitely agree with that. And what's worse is that most of the time, the computer still manages to surprise you. With either the timing of it's attacks or even just the composition and size of it's army. I've had a few matches go with a steamroller AI either attacking 15 minutes in or even later. And having huge armies coming at me, or sometimes just wave after wave of not so huge armies. The psychological factor is pretty impressive considering the problems with the game elsewhere.