1) Mac OSX 10.3 Panther. After the first versions, it was a very beautiful revolution, really sleek and snappy on my dual G4@867Mhz and on my iMac G3 tray loading 266Mhz comparing to 10.0-10.2
2) Mac OSX 10.6 Snow Leopard. The last "old school" Mac OS X, it gave a better feeling to me than Lion and ML.
3) Mac OSX 10.8 Mountain Lion. More stable than Lion, solid feeling, but something of undefined is missing. Maybe too "iOSified", but this is the new Apple way.
I have to say that, although i don't regret old times, i think that something from the feeling from iOS9 is still missing.