The transport bar/ Panel.
In very early versions of Cubase this was more of a rectangle and was changed many versions ago to be a thin bar. As it is now its very unusable filling the screen left to right as an unorgansiable sliver. In 2026 a customisable resizable panel is surely possible?
The Transport Panel that disappeared from Cubase as of version 10? You’ve got my vote.
Here’s my rather long rant from back then:
(for some reason the animated GIF I did to showcase the monstrosity no longer works in that post)
I agree. I complained about the change a lot when it was introduced in Cubendo-10. Even now in N15, it is still impossible to get the entire Transport panel to be visible without moving it, even with my wide screen. I never could see any logic for making the change, other than change for change’s sake. Most frustrating 
I support this, as well. It’s a case of something made more difficult to use instead of easier…or simply not needing to be changed.