I mix it up mainly using our sponsors. Smokingpipes.com and P&C offer free shipping for orders over $95 and $79. Tobaccopipes.com is another site that offers free shipping for orders over $75. It depends on who has what in stock and what may be on sale at the time.
I'll buy from anyone that has what I'm looking for, but my best experiences have been with smokingpipes.com. They reliably send everything I order, they do it timely, their packaging is top notch, and their prices are usually competitive if not lower than others'.
My go to is Tobaccopipes.com They still don't charge state sales tax while most of the others do. Probably buy an equal amount from 4noggins, P&C and smokingpipes from what I don't buy from tobaccopipes. Have never found fault with any of them.
I always go with SmokingPipes. They usually have a great selection, the shipping is free at $95, and the people are really good. I've met Kaz and Shane at the West Coast Show a few times. Awesome guys to hang out at a pipe show with and really good to do business with. SP is also doing deals a lot and if you work up to at least Silver level they do a good number of promos for that level of customer. Right now at Silver I get 5% of all pipes and I get email notices beforehand on some stock ups from time to time.
In addition to what already mentioned I would add Mars for blends more difficult to find in the major sites and Sjnyeco (in Switzerland but they ship to USA) for the SG/GH plugs and ropes not available in the USA.