Resolution is not the only component that determines picture quality. If you’re looking for the best looking version of the original theatrical cut of ESB, Despecialized 2.0 is currently it, imo.
DNR stands for Digital Noise Reduction. It’s a means to clean up and smooth out a noisy image (think about a worn out VHS, all that fuzziness is called video noise). When applied to heavily it can cause lots of problems. In the case of the Dreamastered versions it erases many fast moving things like laser bolts and smears or distorts others such as people running.