Green Arrow is supposed to be explicitly a socialist, cause he's based on Robin Hood, see. How true this is is up to the writer, though. Sometimes he just talks a lot of shit, sometimes he's shown to be a liberal mayor who goes out of his way to help the poor. He's not a great example, though, because as a superhero his actions aren't generally very liberal.
The issue here is that vigilantism, and taking the law into your own hands, is more-or-less an inherently conservative act, and the way that superheroes typically portray it is doubly-so. The idea that some people are just inherently bad and will continue to be bad is very conservative, the idea that the best way to deal with problematic people is to continue to throw violence at them forever is fascist. Some of the better Batman stories acknowledge this: how much of a good guy is he really, is he causing some of the problems he's trying to solve by being violent, how different is he from the people he goes after, etc. etc.
I'd say, of the big name superheroes, the most actually liberal one I can think of is the Flash. Again, like with all superhero things, this all depends on who is doing the writing (and also which Flash it is, I guess.) When the Flash is written well, though, he's got an underlying belief in the humanity of his foes, and although he's always ready to solve the problem by punching it or running really fast around it, he almost always tries to help them or talk it out with them first. In various groups, Flash also tends to be the hero who stops the other heroes from going too far. This scene is
maybe the most flash scene ever, but it's also, to me, expressing a pretty liberal way of how to deal with supervillains: by properly finding out what the underlying issue is (in this case: he has a mental disorder) and solving it (making him realize he needs help) and even working together with the villain to come to a solution (trusting him to turn himself in).
He's still using his powers to engage in actions typically reserved for state actors, and again, solving problems by punching dudes is an inherently conservative action, so it's not like he's a perfect liberal or anything, Still, Flash ends up doing it about as liberally as possible, probably as a result of him generally being written to fill a lighthearted tone in comparison to other heroes.