The ones that I have personally used and would vouch for:
Northern Brewer; used them the most, never any complaints
Austin Homebrew Supply; only used Forrest a couple times, no complaints, he's around here all the time
Brewmaster's Warehouse; another HBT regular, placed one order that I can recall, no issues
Williams Brewing; only used them once (oxygenation kit), no issues
MoreBeer; used them once, a small issue they took care of, appreciate that they helped us raise some money for Babalu87, an HBT regular who's been stricken with EEE
MidWest; haven't used them myself, but have heard nothing but positive things about them
Who am I forgetting... frankly, of all these major vendors, I've had one tiny (resolved) issue. I tend to like Northern Brewer, as they seem to get me my stuff the quickest. Really, any of these shops should be fine.