Photography is all about seeing, and the view when seeing. Thus... is it not odd that above replies seem to leave out the size of the film/accumulated sensor area ??
- Each focal length represent a particular view, as related also to the film or sensor area.
For panoramas I particular like the 617 format (proportions) = 3:1. The particular focal I like with that format is 90mm and that is my only lens for my Fotoman 617. On 35mm terms that equals to 19mm on width or 37mm height. This is just a tad wider than my equivalent favorite focals of around 40-45mm and 21-22mm that I prefer for my other cameras (non panoramic).
For 2:1 proportions, as in 612 images a 65mm focal appeals most to my eyes, which equals 19mm width and 27mm height, while if I can pick a second focal for 617 that would be a 180mm.
I should add that the reason I like 3:1 is the striking/surprising width both horizontal and vertical such slices of capture can cover. While the 2:1 indeed has a stronger impact, I find the 3:1 one more interesting but that is personal taste.
Actually, if you look at classical focal lengths for panoramas you will find they often concur with above choice of focal lengths.
Simply put, the focal length in relation to the film/accumulated sensor is one of the fundamental parameters in photography. The other is the accurate composition. Thus, if you do digital stitching, have you considered buying a panoramic viewfinder to arrive at better composition? See http://www.fotomancamera.com.cn/accessories.asp
(as a parenthesis, Fotoman China is still in producing cameras, while Fotoman.com is not). Alternatively, if I recall correct Ben Rubinstein above uses his zoom to first compose 2:1 images in half the frame, then to re-zoom for 2x magnification of 2-3 stitches to 2:1 ratio, an interesting concept.