Wich characters who is not,O.Sagat, Boxer, Sim or Claw does better against them.
Maybe Honda?
Honda does fine against Boxer and Claw, but Sim and O.Sagat can be hard if they keep him out.
Chun does well against Boxer and Sim, and does OK against Claw, but has a tough time with O.Sagat.
By experience, Claw do well against Sim, OK against Boxer and O.Sagat (50/50 against good Boxers), but have a hard time against Honda.
But I think Boxers should give more trouble to Vega than does Honda. I dont see any really good to see who does better against Calw, Ill look for videos.
Honda and Bison are the two hardest matchups for Claw.
I’d say out of those, Ryu, Chun, and Dictator are your best options. Honda loses too easily to Sim and O.Sagat.
Ryu has about an even matchup with everybody on that list except Claw, and a good Ryu will still beat a good Claw about 50% of the time. O.Sagat can give some Ryu players trouble, but again Ryu has the tools to win easily enough.
Dictator has trouble getting in on O.Sagat, I find. RTSD and win that matchup, it’s a bit ugly. Sim is kind of ugly as well, but again not too bad. Claw rapes him, but I don’t see Boxer winning that matchup terribly often. Maybe I just haven’t seen good enough Boxers?
Chun…well, I’ll let NKI fill you in.
There’s also always Guile. There’s ALWAYS Guile. 
Honda loses to Sim? I figured Sim had the edge, but I always thought it was pretty even.
It’s similar to Gief and Hawk, in that if Honda gets in on you have some trouble. But if you keep him out then he can’t do much to Sim. With that being said, Honda does have more tools to get in on you than Gief/Hawk do. So, it’s certainly not an absurd matchup. But if you play a cautious Sim, then I think he has the edge.