As much as it pains me to do so, I have to vote Panther on this one.
Although I use Tiger and I like it alot, it's been, imo, the sloppiest version of OS X to date. 10.4 rolled out with numerous little bugs present, and, even though none of them were really show-stoppers (at least for me), nevertheless there they were. It took until at least 10.4.4 to get a scrolling bug (when viewing jpegs) in Preview fixed, and it's still not perfect. Spotlight seemed to really get it's game going about 10.4.4 as well. Prior to that, using the Finder's implementation of Spotlight was a disaster, type a search and pray that it doesn't lock-up the system. (Although system-wide spotlight, i.e., from the menubar, never exhibited that behavior oddly enough.)
The cursor that appeared for resizing columns in Finder windows that went missing in Tiger still hasn't reappeared as of 10.4.7. It's basically a lot of little things that are more fit and finish oriented that don't adversely affect basic, essential OS functions/actions, but I never noticed this sort of stuff happening to any great degree in Panther's lifetime. From a casual user perspective, updates to Panther seemed more like polishing the jewel rather than drilling down endless little bug fixes. (Except for the firewire disk bug in 10.3.0 that hosed the data on external firewire drives upon OS installation. Oops...)
Well, the good news is Apple gets to do it all over again with Leopard. Let's see how many lessons they've learned.
Edit: PS, don't even get me started on Quicktime 7