I'm as happy as a clam at high tide with the two lenses I already have:
Summicron 35mm ASPH, and
Summicron 50mm (version IV).
I've been hankering after a longer lens for portraits, so if the market ever quits "correcting" I'd test-drive the Summicron 75mm APO and Summicron 90mm APO. Based on comments and photographs posted on LVL, I think it would be a tough choice.
I realize the 90mm might be contrary for my preference for compact, light lenses and a narrow range of focal lengths for more subtle changes in perspective. However, I have plan to use the longer lens for reference photographs to use in portrait painting. I would use it with a tripod and ample lighting, and I want razor-sharp detail at the subjects eyes and undistorted perspective (ie. no "flattening") of the head and facial features.
If I could add a fourth lens, it might be the Summicron 28mm ASPH.