My favorites:

GEM + Ventura Publisher http://www.typewritten.org/Media/Images/ventura-publisher-1....

Viewpoint http://www.typewritten.org/Media/Images/6085-viewpoint-2.0-p...

AUX http://www.typewritten.org/Media/Images/aux-3.0.1.png

It's suprising at first look that GEM tops my preferences but I recall having a very fond time on the Atari ST 520+. It had one of the best b/w monitors and TOS+GEM was orderly and uncluttered.

Only preemptive multitasking and per-window menus were missing. As a plus, the OS was in ROM, so boot times were <1s.