Powder Valley for any HazMat stuff. They're component prices are very good, so the $20 fee doesn't hit as hard. Also, they seem to keep a good inventory and their response time is fast.
Midway is the old standby. Prices are usually good. Website is excellent. Customer service too. My only beef is with shipping prices, but if you order enough stuff, it becomes irrelevant.
I like Brassman for once-fired brass. Nice people, solid product at good prices.
Grafs and Wideners are OK for a few niche things, but I don't use them much. I use Wideners for SS109 bullets, GP11, and occasionally powder if Powder Valley is out, but that's it.
Natchez seems a little high, so I don't use them. Never used Midsouth, so I can't say.