Here is the question - if you were stuck with only one lens of one focal length which one would that be. I am only talking about focal length here in terms of angle of view - not the real focal length. For arguments sake lets discuss this in 35mm equivalent terms since everybody is familiar with that and most software and cameras will do the EXIF conversion for you. I guess the question of both your mind's eye and what you think would be best in some combination of what focal lengths you actually use the most as I mention here as an interesting study for you to do some introspection.
For me - it has got to be a 50mm both in terms of how I feel and the reality that seems to creep into my actual use based on hard numbers, now that I can easily get there. How about you? Feel free to include other very specific conditions that are must haves for you in comments - I am always interested in what people think are essential qualities - like maximum aperture, no-or-low distortion, wonderful bokeh, whatever. Try not to pick lenses that don't exist in reality if you comment…