For simple, Larry Klein's playing on Boys of Summer is nice and understated, but requires precision. For nice and busy with a challenging outro Pino on Paul Young's I'm Gonna Tear Your Playhouse Down. You could actually do worse than just study Pino's '80s work with: PY, Go West, Joan Armatrading, Gary Numan, the list goes on. Of course much of the early Stones' work was done by Bill Wyman on a fretless. For me, although he has already been mention, Boz Burrell is often overlooked, as he often avoids the slides and vibrato we usually associate with fretless.