There are three ways to divide up Tiers in Marvel vs Capcom 2: by overall team value, by assist value, and by solo value. The following list is the commonly accepted tier list, and represents evaluation of characters garnered from years of experience and play by masters at those characters. Thus while certain characters do not often get seen or represented well, they really do have strong advantages - or disadvantages.
Tiers: Overall Value to Team
God Tier
Sentinel (MvC2)
Storm (MvC2)
Cable (MvC2)
Magneto (MvC2)
Is is generally accepted that Sentinel and Storm are slightly above Cable and Magneto within this tier, but this is a very very slight difference.
High Tier
Cyclops (MvC2)
Iron Man (MvC2)
War Machine (MvC2)
Doctor Doom (MvC2)
Strider (MvC2)
Spiral (MvC2)
Dhalsim (MvC2)
Blackheart (MvC2)
High Middle Tier
Psylocke (MvC2)
Captain Commando (MvC2)
Tron Bonne (MvC2)
Middle Tier
Cammy (MvC2)
Colossus (MvC2)
Gambit (MvC2)
Iceman (MvC2)
Juggernaut (MvC2)
Marrow (MvC2)
Mega Man (MvC2)
Omega Red (MvC2)
Rogue (MvC2)
Ruby Heart (MvC2)
Silver Samurai (MvC2)
Spiderman (MvC2)
Wolverine - Bone (MvC2)
Wolverine - Claw (MvC2)
Venom (MvC2)
Low Tier
Akuma (MvC2)
Ken (MvC2)
Ryu (MvC2)
Charlie (MvC2)
Guile (MvC2)
BB Hood (MvC2)
M.Bison (MvC2)
Captain America (MvC2)
Hulk (MvC2)
Jin (MvC2)
Jill (MvC2)
Felicia (MvC2)
Morrigan (MvC2)
Sakura (MvC2)
Sonson (MvC2)
Zangief (MvC2)
Bottom Tier
Amingo
Anakaris
Chun-Li
Hayato
Sabretooth
Shuma-Gorath
Thanos
Dan
Kobun