Raid,
A 50 is always my go-to focal length. If I'm intending a traditional portrait the prewar 90 elmar goes on instantly. The 85-105 range just does the best job of isolating the subject if you're doing a classical western style portrait. OTOH, if you want the environment in the portrait, then I'd argue for the 50's flexibility. Mr. Adam's picture of Stiegleitz coming down a staircase (Contax I & a Sonnar, IIRC) is just one example of how it can be useful. I'd argue in the 50's favor over wider in almost every portrait case... no, make that every case. I really can't think of a "Portrait" done wider than 50 that I like. Huge amounts of street shots and the like are marvellous, but those aren't portraits the way I see that word in the world.
I do intend to get a 28 for my 7 & CL (prob the CV 28/3.5) but that's really only for when the location is too limited for the 2 foot zoom to work with a 50. Really, pretty close to 99% of my shooting is done with whatever is a "normal" lens on the camera in question. You're obviously talking about 35mm cameras, so the 50 is what I refer to here, but on other formats, I'd still prefer a good prime normal lens if I can only have one. When I stray, I'm more inclined to go long than wide but I understand that's an unusual preference around here 🙂
Hope this makes some sense,
William