Oh my. Tough question. I strop on my Kalamazoo 1x42 machine, I have two 11 x 3 stropping bases for hand stropping, and several different ones for the EP.
For the machine I have leather, cloth, and felt belts. I have enroute some linen belts. So far on the machine my fav ending spot is Ken's 1 µ cBN emulsion on leather.
For the HA strop bases I have in leather; cowhide, split cowhide, horsehide, and kangaroo on glass. Non leather I have balsa, felt, and "nanocloth" on glass. For most routine maintenance of my kitchen knives I use two balsa strops, one with Ken's 3 µ BC emulsion and one with Hand American 1 µ BC paste.
For putting on the strops, I have way to many choices:
Ken's emulsions: 3 µ BC, 1 µ cBN, and .1 µ cBN.
Ken's Sprays: .75 µ cBN, .125 µ cBN, and .1 µ cBN.
HA Sprays: 1 µ diamond and .25 µ diamond.
HA pastes: 1 µ BC and .5 µ Chromium Oxide.
Soon I hope to add some pastes to the mix for use on belts with my machine. I have been playing tag with Ken on this but his sense of mystery seems to be forcing me to go elsewhere for it? We'll see.
When I apply the sprays to bases I use a sheet of copy paper under the strop to catch the overspray. I have even used that paper for stropping with success too. Just trying different combos is half the fun.