Most brands have gems in their line up...and also some dogs. Canon makes some of the best lenses available, but also so pretty crappy ones.
Sigma & Tamron also have very good lenses and some pretty bad ones as well. So you have to make sure that you are comparing apples to apples, not apples to oranges.
At the top of the range, most will tell you that Canon's lenses are the best (for Canon cameras). The L series lenses are very very good...but you will pay a lot for them.
You can get lenses with very similar specs, from Sigma or Tamron, but at a price that is often half that of the Canon.
Specs don't often show image quality (or build quality) so there is the difference.
Just how much better the Canon lens's are...is up for debate. It might be 5% better or it might be 40% better. You have to decide for yourself, whether or not that extra bit of image (and often build) quality is worth double or triple the price. For some people, it is. For many, it's not.
So to sum up...you can get very good lenses from off-brand companies, at pretty good prices...but if you want the 'best' tool for the job...stick with top of the line lenses (and pay a lot for them).