Window Maker/AfterStep were my all time favourites in GNU/Linux world.

I used to be in the GNOME camp during its early days, even wrote a tiny article to The C/C++ User's Journal regarding Gtkmm, nowadays I rather use XFCE.

The original fvwm also holds a special place, that was the first I used in GNU/Linux, back in 1995, and I got to customise it quite a bit.

SOM was great, it also supported implementation inheritance, and had metaclasses concept as well.

I like COM as idea, I dislike how badly Microsoft keeps rebooting the developer experience, and isn't able to provide modern toolig as easy as it was from VB 6, Delphi, C++ Builder. For something that has become the central mechanism how Windows APIs are delivered.