My best party member by far is Butters, mainly because you can use his Prof Chaos powers in the beginning of each battle which can give you a leg up. Very helpful if you are horribly outnumbered by more powerful enemies. I just finished the mission in Canada, which was a great throwback. I started out as a fighter for my first time through, but I look forward to playing again as one of the other classes and also working more with other members of my party to up the challenge a bit now that I know how everything works.