Part of the IQ equation is how much post processing you are comfortable with. I've owned my DS since 2005 and only used the two kit lenses for the first 4 years. Last year, I bought the Sigma 10-20mm. The IQ of this lens is noticeably better than the DA 18-55 and DA 50-200. However, since I shoot in raw and enjoy post processing, the end product from the Sigma lens is about the same as from my DA's. It just takes a different PP workflow.
I recently picked up a used Promaster 20-200 autofocus lens (Built by Tamron). I give it a slight edge in IQ over my DA's. But after PP, the end product is very similar.
Promaster 28-200 at 200 mm with mono-pod.
DA 50-200 at 200 mm with mono-pod
Both shots were taken at ISO 1600 on my DS. The Promaster shot was from earlier this year. The DA 50-200 shot was from 4 years ago. My camera and post processing skills have improved between images.
Tim