I'll throw in my 2 cents, for what they're worth.
I have owned / used the Contax 45mm, DR 50/2, and the Zeiss 50/2 Planar.
Personal opinion:
The Contax 45/2 is a beautiful lens. Sharp, virtually no distorion, nice contrast. It definitely has a pleasing 'modern' signature.
The Summicron DR 50/2 exudes quality construction and finish every time I use it. However I have been disappointed when I've shot color with it -- namely for the reasons other astute RFF'ers have mentioned above: tendency to slightly flare, and lower contrast. I think this lens is fantastic at black and white, but only so-so for color applications.
Risking being called a heretic, I also put my vote behind the Zeiss 50/2 Planar. Every time I use it, I am extremely happy with the results. Beautiful out-of-focus areas, very sharp, nice contrast, easy to use -- everything a modern lens should be. I have gotten more 'wow' shots from this lens than other lenses I have owned or tried.