My #1 subject is portraits, and I gravitate to either 75 or 35 depending on what I want to emphasize.
But since I got a 28, it looks like the 28 may, if I learn to exercise care, supplant the 35. So that induced GAS between the 28 and 75, and of all the 40s available I decided to try the 40/2.8 Rollei Sonnar, largely because Huck Finn has great things to say about its use for candid portraits.
As far as 40s go, I remember Roland did a test comparing the summicron and the nokton sc, and I thought the nokton was better in virtually every category: flare, sharpness, color, and yes, bokeh.
I was tempted by mabelsound's reports on the Pentax 43/1.9, but it looks like an ergonomic elephant in LTM (mabelsound was using the SLR version), whereas the ergonomics of the Rollei are supposed to be really fine.
by the way, there is a silver Rollei Sonnar on ebay right at this moment😱!