If you can afford it, get a Canon L-Series lens - well worth it's money if your want you pictures to look professional.
Personally I own a Canon EF 70-200mm f/2.8L IS USM (among a couple others) and I'm more than happy with it. But it cost's about $ 1600 or so in the US. If you don't need the 2.8 aperture, the f/4 (without IS) is about $ 600.
Pictures taken with my Sigma EX DG don't look anywhere as vivid and focusing is slow as hell compared to Canon's USM lenses.
If you want to see what other people are using and think about various lenses,
fredmiranda.com is a great place to start. Just go to their
reviews and start reading...