I like the look of Captain America's Robot mode. His vehicle mode looks alright, just the colours bring it down.
Carnage is a pass, a big pass. As much as a love the character, I can't stand the mould, so having Venom is more then enough.
Thor is also a pass. He look ok in robot mode, but the colours clash far to much in vehicle mode. Look like an acid trip
I don't know about War Machine though. I haven't had a chance to play with Iron Man, and I am still a bit meh about him, so I don't know about War Machine either