I don't know if I'd categorize Korra as a super hero cartoon but it is definitely a strong contender only three episodes in.
Young Justice is a good solid entry. It's starting to grow on me though I have had some problems with the mentioned angsty/seriousness and level of threats they deal with.
I like Generator Rex, it's just a fun adventure show with a good dose of humor. It doesn't always wow me but it's usually a decent watch. Ben 10 is similar in tone but never really pulled me in.
I just started Ultimate Spider-Man having missed the premiere. It's odd, the way they mix in the humor and the meta sometimes feels awkward. I like that it seems to draw from old American toon styles instead of just anime. I'm still on the fence though.
Avengers seems to be what I perceive to be the standard Marvel animation style but is OK. I prefer the "house style" of Marvel for comics but DC for animation. Despite my comments for Ultimate Spider-Man I do like that they went for something different.
I couldn't really get into GL but I'm thinking of trying more recent episodes to see if maybe they've found their footing, some things start rough.
I dont know if Thundercats fits in here or not, though if Korra does... I do like what this show is trying to do so though I don't think it always works but it's enough for me to pop it on the DVR queue.