To save cost and weight, I started out with a pretty wide spread in focal length: 32, 90 and 180 and since I shoot very slowly anyway, I've been able to get by with stitching when slightly wider shots were favourable with each lens.
Lately, because I wanted to shoot interior architecture (often in small spaces) I've added the 23, worked like a charm for that purpose, but haven't used it for anything else really.
My most used lens is definitely the 32, because I shoot a lot of cityscapes, however, I found when shooting landscape, especially big scenery, the most used lens becomes the 90, followed buy the 180 (and both handle stitching effortlessly because of their large image circles). I found that a 6-image stitch (rise and fall 15mm, left and right 18mm) with the 90 gives me a similar field of view as a 50~60 lens (not scientific, just eyeballing it), which I do find for my type of shooting more convenient than having the carry an extra 50mm lens.