Cap, by a longshot.
I liked Thor 2, but it wasn't edge of the seat thriller, and as we saw in AoS, the events of Malekith's invasion of Midgard got all wrapped up in a neat little bow about a week after, with Son of Coul's team cleaning up debris.
Iron Man 3 (I am still of the opinion) brought nothing of value to Phase 2. A botched Extremis story, a very botched Mandarin story, and, AIM's entry into the MCU botched in an unforgivable way, makes Iron Man not only the worst Phase 2 movie to date, but also the worst MCU movie period. (Even worse than TIH) Add to that the fact that as we know it right now, Tony has no Iron Man suits of armor, it creates some issues for having Iron Man in Avengers 2, without having story to back up why he has armor again, if of course he has armor in A2, which having only Tony (and no armor) in A2 would be a terrible mistake in my opinion.