If you like the 35mm FOV on a film camera, I would go with a 24/25 (you can almost use the whole VF as "framelines". The new 25/4 CV Skopar is a good choice for a slower, more compact lens, the only issue on the Epson will be a bit of vignetting (but that's to expect). I bought one for try it and that's the "walkaround" lens of the moment, it plays the role of my 35 Summaron on my M3. If you need more speed (and may pay the price), I would go with the Zeiss 25 (I have the 21/2.8 and the 25 must be as good, at least... 🙂.
If you lean to the 28mm look on your film cameras, then there is no clear choice: a 21 is not enough, and a 15 wider than you would probably like.
As a personal note, I bought the 21/2.8 Biogon to use as my walkaround lens but (in spite of very, very good image quality) I'm using it less and less, especially for 3 factors:
1) Too bulky - and I don't need 2.8 90% of the time when "walking around"
2) External VF mandatory - extra logistics
3) Too wide, but not enough... 🙂
So... I usually "walkaround" with the 25/4 on the camera and the 15mm on the pocket: if I'm going to use an external VF, at least let's go really wide... 🙂
If I need more speed, that's another story. At night, my walkround lens would be the 28 Ultron (now replaced by a 28 Summicron ASPH - I'll post on that) or the 40/1.4 Nokton (or if I'm into weight lifting, the 35/1.2 Nokton. Really.).