Same:
1st Colt.
NO ONE knows more then Colt, and they have all the parts and special tooling. Their work is top quality, turn around and price are excellent.
2nd Pittsburgh Handgun Headquarters.
Pittsburgh used to be Colt's overflow for warranty work. They also have a good parts supply, turn around and price are good, quality is tops.
3rd Cylinder and Slide.
Are more a true custom shop for high-end custom gun work and Python action tuning. They do world class work, but, like most custom shops turn around is SLOW, and price is HIGH. They are worth it for custom work.
4th Cunningham
I've heard very good reports on his work, but I can't give a recommendation because I personally haven't seen his work.
Pretty much anyone else is a crap shoot, no matter WHAT they may tell you about their Colt expertise.
These days, Colt's are simply too expensive to take chances with, so Colt first, the others second.