From what I see currently in my ultra-long zoom test here
and my big ultra-wide zoom test there
plus other tests I'd say:
Sigma has the broadest offering with some pretty astonishing lenses, Tamron comes next and Tokina certainly has the smallest offering but some pretty good glas too.
But with these third party manufacturers you're never quite sure about the final built-quality. I had one bad Sigma 10-20mm (see my sad story here
), the next copy I'm currently testing looks much better. So if you buy a third-party lens, make sure you can give it back with full refund if your initial testing shows sub-par performance.
Generally speaking: You get what you pay for, as manufacturing lenses is a business that relies highly on precision in everything you do...
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
, My Pictures
, My Photography Blog