Lead your team. Obviously MM has the most versatility (well, maybe Tony), but this is a very specific thing being asked. J'onn is great, but he naturally defers to many of the other leaders around him. I don't want the problems that would come with a leader being rolled over, and I assume I'd have, ya know, a TEAM of people. So basing it on powers alone seems short-sighted.
T'Challa. Cap's a better field general, but he can't double as an engineer/research scientist to provide his troops the best tools and intel for the job. Between T'Challa, Tony, and Batman, T'Challa has the greatest resources (an entire technologically advanced nation with full control of it's natural resources, and a patron god). IMO it's really just between T'Challa and Bruce, and T'Challa has a slight edge.
What leadership feats does BP actually have, or have these skills just been retconned into him?
I look at Avengers history PB(pre Bendis) and dont recall him having any leadership time, much less leading during any Major events in Avengers history.
I guess I'm assuming that the team will have a tech guy, a big gun, etc. I chose solely on their ability to lead. Because, yeah, T'Challa enters the discussion otherwise.
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Cap. By far. Bruce has trust issues, so he is better solo or leading a small group of people he knows well or have trained personally. T'Challa is also better solo - he isn't even king of his own people anymore. While I'm a huge Panther fan, I actually think Namor is a better king/leader for his own people than Panther is. T'Challa reminds me a bit of Reed - he's just too smart for his own good.
As for Tony, well - Cap has proved to be a better leader over and over again, and I agree with Digi about MM tending to defer.