Smartpak and Dover. Expensive, but 90% of the stuff is top quality.. Also expensive, but still pretty good quality. Horse and stateline tack are okay, but unless you are getting a good brand name item (such as Wintec) there can be quite a few cheap things, in my opinion.
I really like State Line and Horseloverz. I have been using State Line for years and have never had a problem with them. Horseloverz I have only been using the past year or so, but I don't have any complaints yet. They have awesome deals on horseloverz. They were having a midnight sale once and I got a winter blanket that should have cost $160 for $30. It was great.
I used Dover previously, but had a bad experience so I avoid them now. I ordered a show shirt, they said it was back ordered and would be mailed in two weeks, two months later I had not received anything and they were refusing to refund my money because they insisted it was still going to sent out. I eventually got my money back from them and got a shirt from somewhere else.