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). 

It's as much about the room and speaker placement than the speakers - maybe more.

+1 Georgehifi

You can try out Magnepan LRS with generous 60 days return policy.

Planar speakers are known for disappearing act.

I also pulled the trigger for LRS although I have nice dynamic speakers.

Charney Audio Companion Horns disappear with SS and tube gear. Soundstage is wide, high, and deep with superior tonal balance.