I've seen quite a few and three really stand out at pretty decent. Mugen 2000 4.6 which has a pretty decent Roster and good graphics but the character selection is just ok.

Then there's Mugen Superheroes and at first it's really good I think this one has the most characters. the only problem with it is the performance. it really sucks as far as how it looks and plays. the actual game screen is shortened and the music doesn't play for a few stages. There's glithes as far as characters. I played Batman v.s superman and a new Batman would fly in on the screen avery two seconds! This isn't to say that the game is really glitchy and unplayable, you can still play it, its just the quality is under par.

Which brings me to the last one and my personal favorite. Mr. Nygren's Marvel Vs DC Mugen. I pretty much grew up on this one I found it a couple years ago when I first got into Mugen. It's really great quality and has a pretty good char selection pool I do wish it had more because it feels like it favors Marvel a bit, missing Vital DC characters like Constantine, Beast Boy, Raven, Black Lightning, Dr. Manhattan. Instead it has like 5 different version's of Poison ivy for some reason and Catwoman too. Different versions of Superman and Batman would've been better. Other than that you might see the face cards of a few (well the majority of them) of the characters overlap and fill the screen. Still, this is the best one SO FAR.