Kotetsu's got it right, it all depends on the circumstance, but without any other external factors I think that it's safe to say that they WOULDN'T all fight each other arena style. Just like with the howlers, it wouldn't be a straight on fight, there would be a lot of tactics and planning and strategy involved.
And that said, I have to say that Marco would win. Cassie was never a fighter to begin with, and if she's fighting against the others it would make things way worse. She just wouldn't be able to cope and would be taken out pretty quickly. As far as planning and strategy goes, Rachel sucks pretty bad. I'm sorry, but it's true. She'd beat the pants off of everyone in a brawl, but if it's a long drawn out affair she would go down. Jake is an ok planner, but he's more of a motivator - he gets everyone else together and keeps the team going strong. In this scenario, there is no team, so his best attribute is gone. What it does say over and over in the books though is that Marco is the best strategist - the one that can see how to get the end result. So he would find a way to take out all the other animorphs and be able to do it by himself.
I could definitely see Tobias causing some problems though. He always was more of a loaner and would have no problem working on his own, and he could be pretty BA when he wants to be...