If I could work with any living Muppeteer, it would be Caroll Spinney because not only is he a genuinely gifted performer, but he also seems like a really nice person in general. He'd be easy to get along with and would tell great stories spanning his five decade career.
If I could work with any deceased Muppeteer, it would be Jim Henson, for the same reasons listed above. Plus, Jim's ability to play serious AND silly characters would please me to no end. It's amazing how he was able to play Muppets ranging in personality from Kermit, Rowlf, and Cantus Fraggle (serious) to Link Hogthrob, Dr. Teeth, the Swedish Chef, and Ernie (silly).