HIFIMAN - SHANGRI-LA

There's the biggest (artificial) and then there's the most natural sounding stage. When it comes to artificial sounding stages, I would say the Sennheiser HD800S/Hifiman Shangri La Sr have the widest stage whilst the Stax SR-Omega has the most natural sounding stage.

It’s been a while since I had a pair of Stax, but I remember that soundstage as amazing, so I’d say those or the Shangri-La, which are pretty astounding.

There are no perfect headphones. Honestly, there aren't really any exceptionally good headphones, either. Technical capabilities are all that matter and EQing to your personal target matters more than any amount of stock tuning. Whether or not a headphone can handle being EQ'd to your preference is pretty much the last thing in the way, for example you can't get any good bass out of a 6XX or HD800s, it's simply not possible. I could be content for life with my modded and EQ'd HE6se v2 and Shangri-La, but that's still 2 headphones, and while my modded and EQ'd Shangs might slam harder than a stock Focal Utopia, they still can't slam like the HE6(which slam hard enough to snap a Focal headband on their own lol).