It's a close call, assuming of course that each OS is to be evaluated "in its day" (no fair making System 6 go up against Tiger for feature set and capabilities). I went with System 6.0.8, which was the culmination of all that had gone before it, and was a solid efficient little OS. I had an accelerated SE that contained a hardware hack that let me run 16 MB under System 6, and I saved a copy of my OS to SyQuest and years later snagged it back to run under vMac. SFVol, Scroll2INIT, WindowShade, MacroMaker, OnCue, Dayna DOSMounter, Escapade, Flash-It, Carpetbag, DeskZap, Quill, KeyFinder, McSink, 7th Symphony (to play System 7 sound files)...
First runner-up and just a smidgin behind the leader is MacOS 8.6, the next time a truly stable and mature OS was available from Apple. As others have testified, 8.6 was solid as a rock once you got it configured. It, too, was the culmination of all that had come before it, all the System 7 and early MacOS 8 stuff with the weeds pulled and the bugs squashed. As with System 6.0.8, the available shareware for extending the user experience made it something truly spectacular when fully decked out. QuicKeys in its prime, DefaultFolder, FinderPop, OtherMenu, Disk Copy, Stuffit Expander, DAVE, FileCM contextual menu plugin, Snitch, Open With, IceTeE, and (testimony to what you get with a well-matured OS that has good shareware)
still Escapade and Flash-It
Panther in third. Enough of the empty spaces of earlier OS X had finally filled in with various offerings, the speed was up, the Open/Save/Save As dialog boxes finally let you have list view back, and once again an OS that was as fragile as a battleship and as buggy as an anvil. OS X come into its own era.
