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+1 for the Tamron 17-50 - it's my main go to lens on the D7000 and I've had almost no problems with it, just be careful shooting wide open on the wide end because of field curvature.
The Sigma is nice as well, but is a large premium over the Tamron - unless you really need the OS (only reason I can imagine is for video), I'd say stick with the Tamron, great deals can be found on B&S!
One more thing. Get the older screw drive Tamron if you can used, AF is much faster and much more accurate than the newer BIM model.