Most “disappeared” speakers.

Coherency between drivers matters to avoid image shift of notes that span the crossover region.  Most speakers that get this right will disappear if placed properly with respect to the listening position.  The most impressive demonstration of this for me was with a pair of Harbeth M 40.2.  You could not detect a single note, artifact or sound localized to the speaker.  Every sound came from behind them as if on a 3-D virtual sound stage.  The key is to have lots of room behind the speakers, e.g. 5-6 feet if possible. 

i had a pair of crappy advent legacys get stolen from the garage rig.. those are so diasappeared i cannot hear them anymore

+1 to @douglas_schroeder  's posts.

Whichever one I want.

....the effect of sensing the speakers have "disappeared" happens when they are setup properly

Almost any speaker can be made to 'disappear' ... this is also about your system (not solely the speaker).

I haven't heard these in a while, but I have a soft spot for wide-baffle speakers. 

The Sonus Faber Stradivari are excellent examples. While small 2-ways often disappear if properly designed and out in the room, the wide baffle speakers have their own benefits and ease of integration, and are much more location friendly. Plus, I think they're gorgeous.