My favorite closed back ever would be the Kennerton GH40 ($700-$900 depending on where you buy them from). Super warm and bassy. the closest sounding headphone to it would be the Meze Empyrean, so for people that have heard that imagine it with extra bass, and a slightly more dynamic/less laid back sound (also closed back) and you get the GH40. However, I doubt it could ever compete for best closed back. Soundstage is great and detail are really good, but still not a contender for the objectively “best” closed back.

For soundstage though I can say the best closed back I’ve ever heard for that category would by my Hifiman + Drop R7DX JM edition modded by JM Audio Editions ($240~). The soundstage is absolutely massive and sounds wider than all of my open back headphones

My other favorite closed backs would be the Meze Liric, DCA Aeon 2 Closed, and Sony Z7M2