Keeping in mind instructions for this forum, "This is NOT the place to discuss a specific lens or lens line...", and to generalize across film formats, I'll advocate a versatile angle of view: 60°-63° is so broadly useful that's all (61°) I brought on my current vacation.
This is about 24mm for a 1.55x "crop" digital, 35-38mm for 135 film, and 75mm for 6x7cm format for instance.
More speed is always useful but balance that against optical excellence and size. I like optics well-corrected for curvilinear distortions and flatness of field, and that comes easier with more modest maximum apertures. One can't really correct later for barrel or pincushion distortion with traditional analog methods, and I don't much like having to correct it digitally in software. Better to start with a better corrected lens... But other attributes like speed and moving elements for close-focus performance are great too.
It's easier to find higher speed with lenses for smaller-formats, and I think here one has to balance lens speed and film speed. With medium format you don't get the graininess with high speed films, but lenses are slower. If you want to do indoor environmental portraits, for instance, are you better off with fast film, medium format, and an f4 lens... or with a slow film, small format, and f1.4 lens?
In the context of a 35mm film rangefinder camera, I'd like a fast compact 35 or 40mm f1.4 or f2 lens of quality design and construction, maybe a Biogon or Planar-derived model. Ultimately, if this is to be your major do-most-everything lens, then certainly get the best one you can afford, as you'll be using it a lot.
