So in general I'm disappointed in the VA in this game and think it's a step down from UMvC3. Mentioned that many times and that it's one of my biggest disappointments with the game. But I'm starting to try and look for the good in it. Here's the characters that have some decent VA.
Gamora - Like how she sounds, although it helps that she doesn't have a UMvC3 counterpart to compare her to. She sounds cool and confident, and she has some nice sound bytes that I think are going to be memorable in that fighting game way, which is very important to Marvel games. Her "arched shot" or whatever it is she says when she's doing that jumping forward shot is the type of stuff I'm talking about.
Dormammu - Not as good as UMvC3, but not bad and it's growing on me. I like the way he puts more aggression into "CHAOTIC FLAME!", but there's a lot to be said about how bad-ass and almost chill he sounded in UMvC3.
Hulk - Seems to sound fine. MvCI is weird because the general VA direction seems to be more about characters putting more intensity into what they're saying, but somehow they still sound more flat that UMvC3. They sound more just....too straightforward I guess? However with Hulk I think it works. The bump in intensity is good. I like the way his "Gamma Charge!" byte sounds.
Spider-Man - While the dude that did Spidey's VA in UMvC3 (Josh Keaton? I forgot his name) was incredible and maybe my favorite Spider-Man VA, whoever's doing it now is doing it fine, and still carries that spunky kid energy that he should have. I don't think he'll top UMvC3 in the end, but it's good stuff.
Thanos - Sounds fine. Sounds like Thanos would probably sound. Big. Cosmic. Dude. No UMvC3 comparison though.
Jedah - Very solid. The laugh in particular is great. No UMvC3 comparison though.
Most everyone else ranges from worse/far worse than their UMvC3 counterparts (looking at you, Ghost Rider), to decent enough and probably something I'll just have to get used to. I am starting to come around though. In general I still definitely think the VA on the whole is worse than UMvC3 and I can't see that changing, but I'm starting to adapt.