Sweetwater.
 
By a long shot. Phenomenal. 
 
It's not like I bought tons of gear from them - I think I must have bought 2 plug-ins before I ordered the POD HD last night, for a big $200, total.
 
Yet, whenever I send an e-mail to ask information, I get a detailed reply within 5 minutes. If the answer is a bit more complex, my sales rep won't hesitate to pick up the phone and give me a call. Not like I have to provide them w/ my details every time either - they have it on file and retrieve it every time my e-mail address shows up. They're always one step ahead.
 
Heck, they'll even follow up and send a personalized e-mail when I enter a giveaway. They always treat me as if I'd bought a million $ worth of gear from them.
 
Obviously, Guitar Center and Sam Ash have the advantage of being geographically close so I often prefer to go get what I want directly because I'm not the most patient fellow when I want new gear: I want it yesterday. But if I could do business w/ only one of them, Sweetwater it would be.

well, that good. And anything I have bought usually has taken only a couple of days to get. (I live closer but even 3 days is good enough. Have time to settle down or gear up for the gear you bought.
And any  problems and they will take care of it.
First RME Babyface was bad in one channel I think, or I thought. No problem, sent me the paperwork to pick it back up and sent me a new one at about the same time. I think it took only a few days not weeks not months, just done and been using the second one for a couple of years now.
Bought a guitar and some other stuff all works fine. No problems with anything I bought from them.
 
guitar center, well, only bought about one thing from them, and rather just look around, not buy anything really there.
At sweetwater the guitar are not demo-ed or used by others, fresh out of the box when shipped. If a demo they show you what happened (usually a small nick or a small scratch and charge less for it.) And they make sure by calling you up that you are happy. (well I did send a few e-mails about the RME product so it may have taken a little bit longer. Being new to it all, I was wondering maybe if I did something wrong, but then again it was no problem in the end.)
 
 

I can't complain about Guitar Center, they've been good to us, they did send e-mails to follow up when we purchased stuff there - even just for something as small as a SM57... 
 
But Sweetwater really bring that up to a whole different level. I don't feel like I'm speaking with a drop out/aspiring musician with a dayjob in a music store... 

Every time I get something from Sweetwater I add a note to my order.
 
"Don't call me on the phone with a follow up... just send candy"
 
:-)

If you've been a good boy, they will put candy in the shipping box for you. Or maybe it's just me they send it to. I get it every time. But then, I've heard they named a warehouse after me a few year ago.
 
Randy