For me it is the new mixwindows.
I had been banging my head against the wall under the old ‘hardware mixer lookalike’ paradigm for how I would provide a self-mixing facility to be used in the studio. Control surfaces were just too limited in configuration for the faders to directly manipulate cue sends (or that I could find out). And getting just the channels I wanted remotely was proving difficult, notwithstanding that the tiny channel studio cue sends were just too fiddly for touchscreens, and the cue sends being horizontal end-to-end was just not visually comparative enough.
Mixwindows, with visibility and grouping on a per channel basis, are just a breeze to design a workable configuration with.
I now have a dedicated mixwindow, with:
a) Several group channels, including a ‘Me’ for the currently recording, fed by channel sends, feeding a master group sent to a studio cue channel. The main faders of these groups become an touch-friendly performer-adjustable mixer.
b) Transport controls and markers (though they could be more touch sized and be able to be docked to the bottom).
c) Control room.
It has worked out well and has been the enabler for self-recording away from the main Cubase windows.
However, I have made a few suggestions to make mixwindows even more flexible in: