I'm thinking Medium Engineer. It seems like engies have a pretty high proportion of objectives they can complete, and I like the idea of buffing myself for increased damage. And my teammates, of course. But mostly myself.
EDIT: Oh and yeah, The_Laughing_Man is right. There's no limit on what class you can play as and from what I've seen in the videos you should probably change it up on a per-objective basis. Still, I think Engie will be the one I use most or when defending.
@GetEveryone: Just to clarify: I never said RPS was bad. I meant other sites are using GiantBomb like they use RockPaperShotgun, which is a good thing. RPS is one of the best PC gaming sites on the internet and I would never speak a word against them.
Sorry for the necropost, I just didn't want to sound like an RPS hatah.
I think it's indicative of how far this site is coming in the news department that now we're seeing other sites using their scoops, RockPaperShotgun style. I honestly don't remember seeing that happen before. Go Patrick and Alex and such.