I have been lucky enough to use all three, here's how I rate them personaly:
1.Canon
2.Sigma
3.Tamron
Tamron:
If you need fairly quick focusing (especially in somewhat low light), DONT get the tamron, It will sicken you with it's sluggish focus speed, horrible tracking and missed focusing. BUT, if you are using it in a way that doesnt require fast focusing and your in good lighting, go for it, the quality is pretty good.
Sigma:
I have used this the least of the three, but from how much I have used it I can still say I loved it. It is much better all around in my opinion than the Tamron and is actually quite on-par with the Canon. I would seriously suggest this lens for your best "bang-for-your-buck" kind of lens.
Canon:
If you can afford it, go for it! It is a fantastic lens and I use it every chance I get. I plan to get one of my own in the future when I can afford it. VERY fast focus speed, tack sharp, great accuracy, and the colors really seem to pop more then the others.
Just my $.02
Hope it helped
