Not sure on rules from sponsors etc, but I have used
And I really like Briar Blues, Pipestud, and Pulvers Briar for the estate and vintage tobacco needs.
Another good one for a mix is Dave Neeb at mkelaw.
To look at some boundary pushing designs I hop onto pipesart.com just to look at pictures. Scandpipes has some gorgeous ones as well.
And don't forget to check out smokershaven.
I use and like a few US websites, great customer care at 4 noggins, great selection of everything at sp.com. For new pipes, on average I've found the Italian site have better prices, especially on my beloved Dunhills.
I love the detail of pipe listings on SmokingPipes.com and TobaccoPipes.com. I've gotten phenomenal customer service from CupOJoes.com and MilanTobacco.com. PipesAndCigars.com of course has an immense tobacco selection. I had a really clean and satisfactory experience with ThePipeGuys.com as well.
Different things for different occasions.
I like smokingpipes best followed by 4noggins and PC. I used to use PC equally to sp for pipe tobacco and accessories, but since they were bought out by Cigars International, I use them rarely as their prices went higher for tobacco. I never used PC for pipes as their pictures and descriptions are sorely lacking. I also use some small mom and pops stores but I keep them a guarded secret, sorry.
To me, this question is like asking what my favorite grocery store is. The answer is that to get what you need, you're going to stop at multiple places.
Market leaders P&C and SPC are a good place to start.
smokingpipes.com, pipesandcigars.com, iwanries.com, cupojoes.com, tobaccopipes.com, 4noggins.com. That's the approximate order, by dollar amount, that I have used the various online retailers. They each have specialties and unique inventory that is distinct with them.
Pipes and Cigars always seems to have the beat deals on tobacco, especially if you buy a few pounds or 20+ tins at a time. And for pipes... Smokingpipes is good, but it really depends on your tastes in pipes. Mike Law has a lot of really good estates, as well as Pipestud. I also buy from The Briary quite a bit, because ot's close and he specializes in high end and artisan pipes. YMMV