I've never found Marvel vs. Capcom 2 to be that terribly difficult.
Street Fighter Alpha 2 , I think, is a lot harder. Same with Street Fighter 3. Heck, most of the Street Fighter series is more difficult IMO.
Don't even look at Soul Calibur or Tekken if you're looking for super difficult. They're fun, but not the hardest.
Virtua Fighter is technical, but I never had much difficulty with it. At least, not as long as you have good hardware for it. If you're on the 360 and you get dizzied, you're probably not going to be able to recover properly unless you have the right controller. And that can make it a LOT harder.
Meanwhile, I'm getting ready to jump into a huge marathon of fighting games. I own 56. I'm planning to play every single one as soon as I get my fight stick for my 360 (I already have quality methods of playing fighting games on my other systems, 360's the last one and I have a Hori fighting stick on the way). I'll probably give you a bit of feedback after my marathon.
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