I'd say the last versions of CP/M for the Z80 - particularly on the Amstrad CPC128, PCW256 and PCW256. We had a whole lab using PCWs as standard equipment, including one sitting at a 3000V DC offset in a perspex cage (controlling part of a C14 accelerator). CP/M 3.0 did bank-switched memory, so the graphics were switched out and you could copy the user-space utilities in to a RAM disk on startup. It was reasonably easy to write RSXs (analogous to TSRs on MS/DOS), which can be handy on a single-tasking OS.