the ducky and kbc caps are, afaik, made from the same molds. SP has their own PBT molds, and imsto's thick cherry PBTs come from yet a third mold. i have used all three, and my favorite by far is imsto's cherry PBT. one big advantage is that the profile is similar enough that it mixes pretty well with SP's DCS profile, and on top of that, they just feel really good. SP's PBT caps would be second -- their texture is extremely nice on the fingertips. third, but not at all least (it's just a lot of goodness here) is the kbc/ducky profile.
the final contender is leopold's PBT, which is only available on their fc700r series. the caps are _fantastic_, and the board itself is extremely solid, rivaling if not beating filco's best. unfortunately, not generally available in the US yet.