Upgrading from Sigma 17-70 F/2.8-4 HSM?

I have the Sigma 17-70mm f/2.8-4 DC Macro OS HSM. I replaced my Canon 17-85 since the F4 was not that great in lower light situations.
I just don't find this Sigma produces images with as great sharpness and contrast in the photos as my Canon did.

Can anyone recommend me an upgrade for this lens in terms of photo quality?
This would be my everyday lense I'd keep on my 600D, so something with decent zoom would be nice.

Budget really depends, I'd say around the $600-800 range +/- .. I'm fine with buying something used.

I was looking into some of those 17-50 F/2.8 which look like pretty nice lenses. I don't care too much about brand as long as it has great photo quality.

thanks in advance

17-55 is about as good as it gets for a cropper. you get the F2.8, IS and a wonderful range on a crop sensor. you can pick one up used around $850.

it was the best lens i had before going FF, and I definitely miss it.

Either the Canon EFS 17-55 f/2.8 IS or EFS 15-85 IS would be an upgrade. Pick the former if you want a constant f/2.8 maximum aperture, or the latter if you prefer the increased focal range.

crop = 17-55mm is 2.8. Its awesome on a cropper.