Raja's is very popular, as Jim Thompson's house (I think that's the name). I think I also read somewhere that the Chinese trained tailors are better than the other ones, but I don't know how you would be able to tell other than if the person is Chinese.
Don't go to Mr. Tommy's (it's in some mall). I bought a suit there, and to compound the problem, I was in a rush. I didn't like the suit and it didn't fit very well.
Tips:
At least 5 fittings (which might mean at least 5 days there).
Bargain bargain bargain