Which speakers have the biggest soundstage

Speakers that image well and flourish in a large room will have the biggest sound stage because the room is bigger mostly.

OHM Walsh speakers typically image from wall to wall in any size room. The larger the room, the wider the soundstage.

mbls perform similarly and have the deepest and most 3-d soundstage I have heard when set up right in a large room with lots of room behind them.

I've heard Maggies set up optimally in a mid-room configuration create a room filling soundstage. Other planars set up similarly probably do the same.

I've also heard a stereo pair Martin Logan and Focal speakers set up near the front wall holographically fill a large acoustically lively rectangular room quite holographically with much of the sound seemingly coming from the rear of the room

I've also heard Dynaudio monitors set up wide and mid room along the long dimension of the room fill the most of the room with an an accurately defined soundstage from wall to wall.

One point to note that in addition to room size, the placement of the speakers in the room is another big factor that impacts soundstage. Most good speakers are capable of big soundstage I have found IF matched to the room and set up well.

What varies more from speaker to speaker I find is how well or how realistically the speaker places things within that soundstage. BEtter speakers are able to do this better in general. Also, a larger soundstage makes it easier for the listener's ears to triangulate and determine differences in location of the musical elements that occur within the soundstage and together form the music and also to detect subtle nuances captured in the recording.

A large well defined soundstage with good imaging is like watching an HD movie on large format TV and provides the best playing field availble in the home for having a shot at reproducing and our ears detecting everything the recording has to offer.