I dont know who makes the best.... but I have dealt with glasstec and vfn and harwood for fenders/doors etc. I would recomend any of these guys.
There are some to definatly stay away from if you look over this forum.
apparantly there are some unscrupulous fiberglass dealers around that offer cheaper price, then dont deliver, or what you get is unusable without a ton of work.
It is not worth the aggrivation to try to save a couple of bucks on fiberglass parts.