Over several decades I accumulated a Leica system with 21, 35, 50, 90, and 135 lenses. Perhaps half of my photos were taken with the 50 with the 90 being second and the 35 third. The 135 was used little, and the 21 even less. However, other individuals shooting in different conditions will likely have much different preferences. One should first consider this when deciding on focal lengths.
This is excellent advice.
On my whole camera gear what i have is 19 20 24 28 35 45 50 55 58 85 100 135 200 300.
At the end when i take a look at the prints i like (the "keepers"), the overwhelming majority was made with a 50 (or 55) or with the 135. Then some few (3 or 4) with the 200, which is what I use instead of the 135 if i can. In the past i used the 24 a lot for full-body portraits of musicians at close length.
I still have made no keepers with the other focal lengths, save for one pic with the 20mm that I liked!
Edit: expanding on my list, i'd say this is how i feel regarding the lenses:
19-20mm: Really deep wide angle, adds dramaticism. Good for getting close to a subject with a huge background, for example a lady next to a mountain.
24mm: deep wide angle, usually with better optical performance than a 20mm lens.
28mm: general purpose wideangle, can be used everywhere, particularly if yours is a low-distortion one
35mm: general purpose lens which gives images with a wide-angle effect. Not pronounced, but it is there.
50mm: general purpose all around lens
55mm: general purpose all around lens, slightly better in portraits
58mm: general purpose lens particularly good for portraits
85mm: portrait lens, almost general-purpose as well
100mm: portrait lens that is particurarly compact and small. Narrower DOF effects than the 85.
135mm: Much more pronounced telephoto effect than the 100 or 85. Very good for portraits and at the same time good for distant subjects. A compact substitute of the 200mm.
200mm: Excellent for full body portraits, models, distant subjects.
300mm: Never used
