The-Digital-Picture.com is by far the most comprehensive site for Canon lenses and is good for other makes. The shear amount of accurate, well-presented and documented information is awesome. I don't think it is biased, and to be able to review systematically just about every one of Canon's own lenses requires the tester to have access to Canon, B&H etc. I truly admire it for being the product of a consummate professional who works exceedingly hard, carefully measuring and analysing. The actual quality of DPT.com as an accessible website is astounding, and the write-ups are flawless. For me, it is by far and away number 1.
My next favourite is not on the list: slrgear.com. Its graphical illustrations of sharpness via the "blur units" are unbiased and really informative. It does not have the comprehensive coverage of DPT.com, but is still outstanding.
Photozone.de is also absolutely excellent in what it does review, but it has fallen back and no longer has a sufficiently wide range of coverage.
Lensrentals has some wonderful stuff with deep insight, but again is not comprehensive, but what is written are real gems.
Lenstip has a few useful reviews, and is run by nice guys who are rather too nice.
DXO, I can't understand.