Most “disappeared” speakers.

Easy. MBL speakers.
I got my 121s for a steal years ago, but that was because they were damaged in shipping to someone else so they were sold cheap.
Nothing disappears/soundstages like the MBLs that I've heard.
Then again, it's rare to find them that cheap. 

Otherwise, I agree with george, Audio Physic speakers are reliable for a disappearing act (one of my other favorite brands). 

Agreed that almost any, or most decently designed speakers can "disappear."  Though some do it much more readily and successfully than others.