If you are looking at f2.8 zooms, here are my opinions and generalizations.
About the same in my view in terms of resolution. Tamron and Canons have a very similar color cast. Tokinas seems a touch colder and Sigma seems a touch warmer. Sigmas tend to engineer their lenses to focus very close which is very nice to have. Tamron as well but not as close as Sigma. Generally unless you are measurebating, absolute difference in sharpness is not a big deal.
Generally, Canon lenses with ringed USM AF motors tend to perform best in terms of accuracy, low light focusing, and quietness. Sigma with HSM (their version of USM) is almost as good. Tamron and Tokina untill recently by Tamron have not used USM AF motors so theirs tend to be slower and a bit less accurate in low light settings. Tamron imo is the worst, many of their lenses have the MF ring that rotates as you AF and they tend to make the most noise like the 17-50mm f2.8 non VC lens.
Generally, I'd say Canon L lenses have the best feel and build quality with a very tough finish and build feel. Tokinas come second for me with a very Nikon Pro grade lens feel. Many describe Tokina's Pro grade lens as Tank like in how tough it feels and I'd agree. Sigma is pretty good but their finish is very soft and a bit more prone to wear imo that Canon or Tokinas. Tokinas are also very heavy, as heavy as Canon L's imo. Same for Sigma's EX lines. Tamron uses plastic in their lens barrels to make them lighter and their lenses like 17-50 and 28-75 is often smaller than their couterparts from Canon/Tokina/Sigma. The plastic material may feel a bit cheaper but it's the same type used in pistols so they are very tough and have a self lubricating feature to ensure smooth use over long time.
Canon does not include a hood except for L lenses. Their front lens caps suck big time, especially if you leave a hood on the lens since they are not center pinching type. Their rear element cap is generally better than the others in term of use. Tokina, Sigma, Tamron all include hoods and a center pinch front lens cap that makes it easier to use with a hood on. Of those Sigma and Tokina have easier center pinch lens design. Tamron's rear lens cap is the worst, you have to line up the cap with the little red dot to mount the lens. Rear caps from others do not require this. This can be annoying when you are trying to quickly change lenses.
Canon has a one year warranty on all it's lenses. Tokina and Sigma (EX) line have 4 years IIRC. Tamron has 6 years. Many will say this or that brand have better quality control but I've not seen any authoritative figures on failure rates or such from a lens maker. So I can't say a specific maker has the best quality control/reliability. IIRC lensrentals.com put out their own stats and it seems like failure rates where high for specific models more than a specific maker, but you can search that on your own if you want. Personally, I'm not concered as most lenses seem that I've bought worked fine.
Canon's tend to cost the most. I'd say Sigma and Tokina are comparable and maybe Tamron's costing just a touch less than the others. This is however a broad generalization. Canon has a fall and a spring rebate which is instant now days but prices often rise a week or two after the rebates start and often negate much of the rebate. Tamron has a more generous rebate once or twice a year but it's a mail in type. Sigma states they do not have rebates and I've never seen them have a factory rebate. I've also never seen Tokina have a factory rebate program either.
Some nice fast lenses/zooms I'd recommend looking at that combines good optics and value for the money.
Ultrawide: Tokina 11-16 f2.8
Normal zoom: Tamron 17-50mm f2.8, Tamron 28-75mm f2.8, Canon 17-55mm f2.8 IS, Canon 24-70mm f2.8
Telephoto zooms: Canon's may make a better value as the cost difference seems to drop in this category.
Do your own research from forums and various tests. Some good test sites include dpreview.com, slrgear.com, photodo.com, our own lightrules site http://www.pbase.com/lightrules/lenstests.