I voted Kaldheim as I believe most of the settings aren't extensive enough or have other issues.
Strixhaven is confirmed as getting a setting book, so it doesn't count.
Zendikar is fun to play, or aside from Eldrazi and the former Kor empire, it's not that interesting.
Dominaria got depth and lore, but I like Kaldheim better.
I almost choose Amonkhet, but it's a huge mess right now and it's civilization is one small city, that it. Great races though. Needs a revisit.
Tarkir is a possiblity, but it's caught between two timelines.
Eldraine is my second choice, but I find Kaldheim more interesting.
Kaldheim more then any other Plane, besides Arcavios feels like it was designed with an eye towards some day being a D&D setting, something pre 2021 settings weren't designed for.
Zendikar has alot of space, but feels like there is almost nothing but wilderness is in it. Good for Dungeon Delve at best.
Dominaria could get the nod, but given that it is a poorly kept secret that Dominaria : United and Kamigawa: Neo Dynasty are coming next year, and WotC's setting slots are pretty full right now I doubt it will be either of them.
So next trip to Kaldheim and Eldraine I believe will be it, maybe Alara as well.
Ikoria is pretty much just a giant monster manual with a couple of cities.
Kaldheim admittedly could use 2 sets next time to expand and explore it's themes, but it's big, lots of different cultures, races, cities, ruins, realms, tribes, and so on.
I could see playable Haggi Trolls, playable Zombie Nobles, playable Yeti, maybe so others like playable Valkayrie.