I thinking of doing a best of best, using my own guitars as examples. New capos have been few and far between lately. I do not think anyone will introduce something other than minor variations on current neck up and neck down capos designs.
Using 6 guitars, ranging from resonator and baritone as thick necks, 12 string as wide neck, and Seagull Mini-Jumbo, 000m Sigma and Ibanez parlour as progressively smaller guitars, I plan to list the best capos I have found for recording and performance on each guitar. Yes I left out the Martin Statabond neck.
I plan to measure and list the "throat" required for each neck at 3rd fret. Throat being the width and depth opening of capo required to put it on each guitar. (eg trying to get a Kyser on a round-neck resonator snapped the spring) During my years of testing I have encountered more than one capo that could not fit on some guitar at 3rd fret and some that could not fit at 9th fret on 14 fret models.