I hope all of next summer's superhero films do great business, but they're all tough to predict. I can't see any of them being outright flops, but I can see any or all of them delivering anywhere from great numbers to disappointing, middling numbers. At least one of them is bound to underperform just based on the law of averages.
Thor has the best release date. The first weekend of May kicks off the summer movie season and usually yields a hit. The following weekend also currently has little competition. On the other hand, Kenneth Branagh fell flat on his face when he turned his hand to big budget fantasy filmmaking with Frankenstein and the first weekend of May yielded only middling numbers (relative to its budget) for Van Helsing. I think Thor will do at least Hulk-level numbers, but it'll need to push well beyond that to be a bona fide hit.
X-Men: First Class has the benefit of being part of a popular established franchise, although whether franchise fatigue will start to set in with audiences remains to be seen. All of the X-Men films have been very frontloaded in their box office and this'll likely be no different, so a big opening is key.
Green Lantern has the benefit of an increasingly popular name star in the lead and its sci-fi elements and FX are well-positioned to draw moviegoers to 3D theaters. It has a pretty good release date. Fast Five, opening the weekend before it, will likely open big and then drop off fast. It'll have to deal with quite substantial competition, though, from Cars 2 and Rise of the Apes the weekend after its release.
Captain America: The First Avenger has the toughest release date of the four films. Moviegoers may have had their fill of superheroes by late July and there will have been an awful lot of big FX blockbusters released by that point. Moreover, it's being released a week after the behemoth of the final Harry Potter film and a week before Cowboys & Aliens, so competition is going to be intense.