You using digital? If so, gotta get sigma or Nikkor 12-24. Both are not sharp lenses but if you ask me i'd get the sigma. But to share my practical experience, the type of unsharpness in the sigma, you can easily apply careful USM to force out decent sharpness. Don't know how to explain it, but i can force out better/more/useful sharpness (for screen/print) from the sigma 12-24 compared to lousy lenses kind of unsharpness like 28-200 zooms.
Else for this lot, i think Nikon is the best. Followed by Tamron. Sigma 17-35 is optically decent but got some complaints on build quality and stuff, HSM is not that fast so don't be fooled (though it's silent). However, for the best bang for the buck, the 18-70DX would be best and more convienient. All the samples i have seen from the 18-35s you mentioned, i'd not say they are better....just that the 18-70 got some moustach distortion at the wide end (so depends on what are you after). But seriously, the 18-35s, i just read stuff on the net as i was looking to buy then, so don't quote me.