I'd probably pick Ax or Marco.
Ax in human morph would need a
lot of watching ("no, Ax, we don't eat banana peels!") but he's a freaking genius with technology. Of course, getting him to share some of that knowledge would be a little harder, but we could probably make a deal. Say, one hour of lessons for a dozen cinnamon buns?
Marco and I have very similar personalities, so we'd get along just fine after awhile. I'd probably have to prove that I wasn't Yeerked first, though.
I was initially going to say Rachel, but I changed my mind once I thought about it more. She's pretty hotheaded, and I don't relish the thought of trying to argue with somebody who could (and quite possibly would) morph a grizzly bear and take my head off.
I'd be willing to take in Jake, but I don't know how well we'd get along. Especially by the end of the series, he didn't want to talk about anything with anybody, which is something I don't do too well with.
Tobias... sorry, Tobias fans, but he's a little too emo for me. I respect his part in the missions, but I don't think we could live together happily.
I could probably get along with Cassie, but it would be a definite strain on my patience. Tom said it best: "Wars aren't won with clean hands", and Cassie refuses to acknowledge that. I can see her viewpoint, but by the end of the series it was a drag on the Animorphs' resistance efforts.
