rpgnow most, followed closely by dtrpg then Paizo and ENGS, based on how much I have gotten. Considering E23 for GURPS things but I have not used them yet.
Paizo for old edition D&D stuff has caught my eye and monthly gaming budgets recently with their $4 price point old products. DTRPG has a bunch of good stuff not available anywhere else but I slightly prefer rpgnow non watermark pdfs for stuff available at both sites (though it is not a big deal to me).
Mostly it is a price issue for me of things I want. Paizo sells old D&D stuff for a $1 less than rpgnow so I buy them from Paizo, I got the EN critter books at a half off sale here at the ENGS and I load up at DTRPG when they do their holiday or malhavok sales. As long as the pdfs are not DRM which I will not buy/use, the watermark is only a minor issue, though if the prices are the same I will go with the companies that do not use watermarks.