If a robot is playing MvC2 Dhalsim he is godlike and with gauge and the right team guarantees a win via full unblockable set up into death combo into guard break against all 3 characters. Humans are not robots and all the amazing things about sim are less amazing when you factor in the chances of actually hitting them unless you have Khaos’ execution is not at a high enough ratio to rely upon them. I also play Dhalsim in MvC2 and think in theorycraft he’s right there with Sent and IM, in practical gameplay with what humans can do he’s good, but nowhere near Mags who needs to work a tenth as hard to get the same reward.
Viper is not like Dhalsim, all her tools are possible for humans to execute consistently, they aren’t 3 1 frame links to just get 1 attack to chain into another, they’re just very foreign commands that a lot of people don’t have down yet akin to early ROM. In theory fighter Viper has a 10-0 match up against every character who isn’t MODOK thanks to how the match starts + an invincible move that leads into a block string that leads into an unblockable, but practically she is still amazing and useful.
Naw, my execution is pretty bad for someone who plays the characters I do. Despite beating K-beast at the previous Starbase tourney I can’t even consistently rapid fire seismic hammer. ^.^; Haunts has ranted before about how in SFIV my combo was jab jab throw because my execution was that shitty. I’m not that terrible any more, but my play comes more from basics and decision making or else I would definitely have the infinite seismos of doom down. XD
I’ve tried Magneto in this game, he’s easier to use but I find the inherent risk reward he has although amazing isn’t as stupid as Viper who can potentially take next to 0 non execution based risk for massive potential reward. I think Mags is definitely top 8, but don’t think he’s anywhere near Viper once people actually start to use her.
Also to revise my previous tier list, after some thought I’ve come to agree with people about the validity of Storm as a character, primarily thanks to the fact that her damage issue is solved thanks to the DHC glitch. I was wrong about her and thus changed my opinion. 
Each letter is not necessarily in order except for S+
Unknown tier: I don’t adequately understand enough about these characters to tier them.
Jill: Potentially very good, I don’t have enough understanding of what she can do. ©
Shuma: Great assist, interesting tools on point potentially strong (C+)
MODOK: I’ve seen some people use him to great effect and he seems to potentially have good match ups against some notable characters (Wesker, Dog, Viper, Wolverine)
Thor: I do not have the information required to adequately say what he can do, precursory glances made him look bad, but I haven’t gotten enough lab time to check.
Spiderman: I’ve heard from many people that he’s amazing and he has a lot of interesting tools, he can definitely open people up and do damage just unsure exactly where he should be placed.
Viewtiful Joe: I see a lot of potential though I fear a lot of the characters with dominant tools have very effective counters to it.
Hulk: A lot of people I respect say he’s being slept on, I don’t see it but I’ll take them at their word on the subject, regardless I think he has bad match ups against a lot of good characters.
Special case tier: Other factors are important team related factors may outweigh their ability to function on point.
Haggar
Tron
S+: Other than execution I view these characters as having minimal downsides and very few bad match ups.
Dark Phoenix
C.Viper
Dante
There is a significant gap between here and everywhere else
A:
Magneto
Ammaterasu
Taskmaster
Wesker
A-:
Doctor Doom
Dormammu
Zero
Storm
B+:
Trish
Spencer
Wolverine
Felicia
B:
Chris
Akuma
Spencer
X-23 (If she touches a character the character dies, and once she gets in that’s not too difficult depending on the character and assists, I really feel that a lot of teams completely keep her out however.)
C+:
S-Skrull
She-hulk
C:
Chun-Li (May be higher, her rushdown is amazing just everything else isn’t.)
Ryu
Deadpool
Sentinel
C-:
Tron (Yes, I know she’s on here twice.)
Captain America
Iron Man
Morrigan (May be higher, though she has some large fundamental issues, may have good matches against some important characters.)
D: These are among the characters I feel with their given tools can’t compete once people start doing things effectively or have such a crippling match up as to prevent their use.
Arthur
Hsien Ko
Haggar (Also on the list twice)