Best Ski sock for looser boots?

You should see a bootfitter. Thicker socks won't be the solution for the long term.

out of everything it could be, it’s not the sock

Like other posters said, it sounds like a bootfitter should be your long-term solution.

In the meantime, I'd recommend a thick merino sock. I absolutely love my People Socks. They're made in the USA and durable as hell. I've been hiking and backpacking in the same 4 pairs for 6+ years now. I don't ski in them because I prefer my thinner smartwool socks, but they would certainly do the job.

I've heard great things about Darn Tough too. Get a midweight over a lightweight for the added thickness. They're a little pricier than the People Socks, but that lifetime warranty is pretty cool.

Definitely not a sock issue. Did you not try them on in the store with ski socks on? Using thicker socks may provide a little more support but that support is soft so you'll still have movement. Ski socks should not be thick. Only your socks should be inside your boot. Etc.

Also are you buckling up properly?

Advice: take back to shop you were at and see what they can do, depending on if you used them or not and how poor the fit is. Many shops offer free boot work on boots they sell.

As for socks though, Darn Tough and Bombas are two great brands that both make merino wool socks. Bombas I like cuz they're a wool/polyester combination and are very form fitting and comfortable in a ski boot. They don't stretch out over time. Darm tough is nice because they have a wide variety of socks for cushion as well as warmth. They use merino wool but I find that it tends to stretch out over time but returns after each wash. They have a lifetime guarantee so I'm gonna hold them to it when the times comes. Had my current pairs for 5yrs and still going strong.

Uhhhh all boots fit different you can’t just buy a new boot in the same size and expect it to fit... pls see a boot fitter because a new sock won’t solve the problem