Springfield Armory for me, hands-down.
Once got a pistol back from them for warranty work within 2 weeks, over a holiday weekend to boot, with some extra work thrown in, hadn't been asked for, for free ... Custom work is not cheap, but the Custom Shop does a terrific job. Warranty work is free, and SA is very fast. Good phone service.
Smith and Wesson is pretty good, not quite as fast as SA. Friendly.
Heard some good things from friends about Ruger's recent customer service, no personal experience.
Had one quite positive and very fast response from Taurus (sent me a replacement part, asked for by phone, for free -- got to me within 3 days).
For accessories, Hogue has been friendly, efficient and fast.