Sigma, Tamron, and Tokina were the highest rated when I was working behind the counter.
A friend and I used to shoot weddings together - he used his Tamron zoom and I had my Sigma - dunno how much camera shake he had compared to me but his shots were always a bit softer. About the same contrast, just softer.

If you are able to use Zeiss, Schneider, Angenieux, or Leica lenses, DO IT!! These are the highe-end and luxury brands (but they have the price tags to match).

For web applications (and general low-res finished products) you'll NEVER see the difference. Use what you like and can reasonably afford (don't go for the cheapest lenses you can simply because they will save you money - you get what you paid for).

Once you start using it for printing purposes (sometimes I'm shooting a single image for a 2-page spread, so this is a big deal for me), I stick with Sigma, or OEM lenses (or I just break out the medium and large format cameras).

Hope that helps-
-Lew ;-)