Hey Cleng,
umm.. you didn't mention what kind of shooting you'll be using them for? Or a budget. Or why you short-listed these particular three lenses.
I'm sure they all "work" just fine, but that doesn't mean they are best for you or best in certain conditions.
I actually wouldn't think the Tokina 12-24 would be in the same category as the Tamron or Sigma lens, simply because it is much more of a wide-angle speciality lens whereas the others are a bit more versitile (but not as wide).
I had a little look at your other posts and you mentioned you want to take candids and do wedding photography. Firstly none of these lenses are going to be particularly suitable for candids, but I can imagine they'd certainly have their place at weddings for group and indoor shots.
If you are looking to replace your kit lens with a better quality one then I'd be looking for something with a bigger aperture (smaller f-number) than the ones you listed above. Not only will they perform better in lower light (dusk, indoors, etc) but they can give a more pleasing bokeh (background blur) and generally a more romantic vibe for a wedding.
I can't comment on the quality of the Tamron and Sigma lenses you list because I don't know, but there is another Sigma - the Sigma AF 18-50mm f/2.8 EX DC which would be the obvious choice for me to replace the kit lens for weddings. Our very own Mitch has done a review on this lens which can be found
here.
I actually did replace my DRXT (350D) kit lens, but I chose the Canon EF-S 17-85 f/4-5.6 IS instead of the Sigma for numerous reasons (image quality not being one of them). You can read about my experiences with this lens
here.
If you *don't* want to replace your kit lens (as you mentioned in your other "lens question" post), then I think there is little point in getting the Tamron 17-35 or Sigma 15-30 you list above, as the 18-55mm kit lens covers almost the entire range of both lenses and is f/3.5 at the 18mm end anyway, so you aren't gaining anything with the aperture of these lenses either. That's not to say they are bad lenses at all, just that they don't really do much the kit lens can't do.
Which brings us to the Tokina 12-24, which at 12mm is starting to get a noticable amount wider than the kit lens. I've heard that is a great lens (haven't used one though), but if you are looking at an ultra-wide zoom then I think it is well worth considering. But there are a few others that are also well worth a good look.
The Canon EF-S 10-22mm, the Sigma 10-20mm, and the Tamron 11-18 (I think) are all alternatives to the Tokina. I actually own a Sigma 10-20mm and love it, but Zig had one and wasn't happy with it - he took it back and swapped it for the Canon 10-22. You can read about our experiences and see some pics from both these lenses
here.
But without knowing what you want the lens for or being given a budget it is hard to give any real advice. All I can do is offer a few other suggestions you might want to look at.
Cheers
Adrian
