Graviton, definitely. Kang's powers are all tech-based and, as Iron Man showed, can be overridden. Graviton's powers operate on a much more basic level, so they're harder to counter and pretty much impossible to resist.
As for Thor, I agree, I'd love to see him interact more with Tony and Cap and see them really establish the Big Three concept in the cartoon. But that basically just comes with time. Can't rush it or somehow imply that Thor, Cap, and Tony are "bigger" members of the team when everyone else has been there as long or longer. Cap has to distinguish himself as the leader to end all leaders, Tony has to establish himself as the biggest brain of the Avengers, and Thor has to establish himself as the powerhouse against whom all others are measured. But Thor's role in the show is complicated quite a bit by the fact that the Hulk is a permanent member of the team instead of a founder who leaves after one issue and never comes back for any significant amount of time.
As far as Thor being a tide-turner, though, I think they're doing well with that. He's struck the crippling, climactic blows against both Graviton and Kang now. Sure, I have some problems with his getting batted around by the Hulk and Cap and generally not displaying much in the way of fighting skill, but you can't objectively say that he hasn't been a major factor in several big battles so far. The other Avengers basically just set villains up for Thor to fry the everloving s*** out of on several occasions. I liken him to Superman in the Justice League series: he may not live up to fans' expectations of his power level all the time for dramatic reasons (can't have one character easily beating threats the whole team is supposed to be dealing with), but when he does, it's a pretty satisfying spectacle.