K1000

I have the Raal SR-1a, the AKG K-1000, the Sennheiser HD800, and the Raal Magna (too poor for the Immanis lol). The Raal SR-1a can throw a slightly wider soundstage than the AKGs, but sounds better with the wings much closer to your ears. It’s the biggest soundstage of the above, though I prefer the sound from the Immanis in every way. If you’re really looking to get an experience larger than a headphone’s natural “headstage”, though, you need processing more than novel driver placement. Look into the “Smyth Realiser”. It is a headphone system with a wider soundstage stage than most speakers.

Personally, soundstage "size" isn't the same as how natural it sounds. I genuinely think the HD800 has a larger soundstage than the K1000 does, but the HD800's stage sounds very exaggerated. I do think that open-baffles are the "secret sauce" when it comes to good staging (K1K is my favorite headphone by a very large margin), but the fact that the stage is large isn't what makes it sound better automatically. KSC-75's don't have a very large stage but still greatly benefit from their extremely open design in the same way the open-baffle phones do.

I have owned or heard many of the soundstage champs out there including quite a few mentioned on this thread and the AKG K1000 had the largest stage I have ever heard. Larger than any Stax I've heard, larger than any Hifiman I have experienced, larger than the HD800/HD800S, larger than the Raal SR-1b, even larger than the MySpheres that were meant to replace them, etc.

Has to be my AKG K1000. But I use others more, such as the Ath-Adx5000

Dynamic: Sennheiser HD 800, Sony Qualia 010, AKG K1000 Planar: HiFiMAN HE-1000 v1 (with the deeper cups) E-stat: STAX SR-Omega, SR-009/009S, Sennheiser HE-1. I haven't heard Sennheiser HE90 but that also has a huge soundstage supposedly.