Which speakers have the biggest soundstage

The new Magnepan 1.7s. They will play louder than you think with 40-50 watts of good power.

In my experience, most dipole speakers do an excellent job with soundstage. I've heard great examples with planar, electostatic and open baffle designs. This is a fundamental strength of most boxless designs. On the flip side these type of speakers (in general) have a harder time with bass and microdynamics. There is no perfect speaker. Pick your sound priority and find the speaker design that optimizes accordingly.

woow thx everyone for your suggestions. There're quite a lot of speakers I gotta try!!

Unfortunately I wouldn't add any room treatment nor replace any other equipments except my speakers.

How about the Zu Essence and Tyler acoustic D3 or acoustic zen adagio?

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.

I just auditioned the Gallo 3.5s, which were remarkable on soundstaging and imaging, especially considering their small size (the pictures somehow make them look bigger than they appear in person).

The list at 6k, and it's a bit early to find them used, but it might be worth considering the 3.1s, which could easily be had at your pricepoint (well below, actually).

Sensitivity might be an issue, but I think some people around here are pleased with the results with Gallo's and lower power gear.