The best usability I get is from Windows XP. This is the only reason I keep WinXP still as my main operating system. The user environment does what I expect it to do at any time. 95% of the applications carry out user-interactivity actions exactly like another Windows app would do it. There are tooltips everywhere, great keyboard navigation that will let you move everywhere in case your mouse has screwed up. It is just the 'standard', we like it or not. Can it be better? Possibly. But from usability/accessibility point of view, Luna is the best out there. However, it is not all sugar. The new "Start" menu found on XP is just too loaded with stuff. Programs need an extra click to get into them while it is the most common reason why would someone would click on "Start". The Open/Save dialogs can be better as well by including a drop-down menu for recently-visited places. I hate it when I save something with Paint Shop Pro on the A directory and then I need to save something else on the B dir and I have to navigate manually each time between 5 and 8 clicks, while it can be done with only one.
On usability, MacOSX and BeOS are the second best, both at the same level I could say. They are consistent and OSX offers some new tricks on the play, like quick navigation with the help of Finder, excellent drag-n-drop support, speech recognition & hand writing recognition (not with great performance on these two features though). What Mac OS X lacks though is good keyboard navigation. For example, I get an alert window to save my text file and I can't move fro a button to its next button with the cursor keys. Yikes! (Update: Apparently you can enable this via a pref panel, but it is not there by default) Another thing I recently realized deeply is that Macs are way more keyboard-oriented than the rest platforms, because of the lack of a second mouse button (however keyboard navigation is not as good as noted above). If it was not enough for the CNTRL and ALT buttons to do things like context menus, we also have the OPTION button... Not good for most mouse-oriented users, especially in a period where Apple is pushing their Switch campaign to Windows users. Also, I don't like the fact that Finder doesn't have more options on its context menus or elsewhere, like the "Open Terminal Here" option (hey, it's a unix underneath).
BeOS has great usability. Everything is brainlessly easy on that operating system and it is one of the reasons people who have tried it, like it. The OS is extremely simplistic in its nature (even installing drivers is as easy as dropping a file on a directory - and you probably won't need rebooting either) and its user interface is also simplistic on the way it works. Its context menus make sense and they add great functionality, like the Tracker add-ons (similar to Nautilus scripts), easy ways to copy/move/shortcut files with a single click and has even the easiest way to date on mounting other filesystems! The Deskbar is also easy to use and it does the job adequately. Drag-n-Drop works everywhere! Tracker, the integrated file manager, is awesome too. What I always needed from BeOS though was more context menu functionality when right clicking on some widgets. For example, when having an input box, a text view, or a selectable text item, I want to be able to right click on it and have a cut/copy/paste menu. I always missed that on BeOS, which I know that it never had that because of its Mac-oriented roots regarding its interface (BeOS ran first on PPCs before coming to x86). Also, having the CNTRL key as the default action key instead of ALT, wouldn't hurt either (and I have heard of some big fights about this in the management/engineering departments at Be back in the day...)
KDE has a lot of new features and goodies with version 3.1, but Konqueror (the main KDE application) leaves a really sour taste. It tries to be everything for everyone, so we get functionality from file viewer, to image viewer, to CVS front-end etc., in addition to its two major uses as web browser and file manager. All that may sounds good, but the problem is that you get extremely long menus or context menus with options that have nothing to do with the KPart currently loaded. I find Konqueror abusing the otherwise great KPart technology; it adds extreme bloat to its interface when it is not truly needed, while configuring its toolbars is a pain in the rear (with bugs too). I have outlined my problems with KDE more detailed here, which was later discussed in the kde-usability list. Another problem with KDE is the extremely bloated default KMenu (which unfortunately many Linux distros keep), big icons for shortcut/launchers on Kicker (a default KDE screen doesn't fit well on a 800x600 screen) which makes difficult to distinguish that the K is a menu and the other icons next to it are just shortcuts. Thankfully, work has been done on the context menus on the desktop, but when right clicking on the icons on the desktop we sometimes get options that shouldn't be there (e.g. the Trash's long menu, while it only needs 2-3 options - this has been fixed in the CVS from Waldo Bastian AFAIK). The main problem I have with KDE is its extreme bloat. Cut the fat and suddenly everything will be better. Second grade problems are the choices for the defaults, like the single-click action and the "hook on the other windows' borders" of the window manager. I believe that KDE should leave-in the hook on the monitor's border, but take out the application border hook as it creates a bad impression to the user thinking that "Xfree is slow because when I move my windows around, it is not smooth", even if this has nothing to do with the reality. It is all about perception, UI is all about psychology, and KDE takes an F on that department.
Gnome is more simplistic than KDE in its choices. It goes straight to the point and the applications written for it tend to follow its Human Interface Guidelines (HIG). What I dislike though is the default Application menu bar on the top of the screen. It takes space for a no good reason in my opinion (not everyone adds icons to that bar all the way through the 1600th horizontal pixel, come on). I am more in agreement with Red Hat's and Mandrake 9.1 defaults personally. Gnome also is not all that great when it comes to its Trash context menu, Nautilus is ok but I want a dialog box asking me for the root password when I am in need making a system copy/move. The functionality of the Red Hat's Gnome taskbar is fine, but it feels a bit amateurish, icons in the notification area move by themselves and create unwanted space, the menu is ugly and looks like a potpourri. However, applications like FileRoller and Red Hat's RPM installer application make the whole experience better. Gnome can easily become better than what it is today. Exactly because it is already simpler than KDE, the work required to clean up things, I think, would be less overall. However, I don't understand what took the Gnome project (especially Red Hat) developers so long before they start working on the new GtkFileSelection. It is now scheduled for Gnome 2.4, which comes out at the end of the year. Also, why can't I move the toolbars from Nautilus next to each other and save some real screen estate? Anyway, more here.
Rating: Windows XP 9, MacOSX 8.5, KDE 6.5, BeOS 8.5, Gnome 7.