Ideally something thick, non slip and that doesn’t end up with a two inch floppy bit hanging off the end of your toes! 🤣
I just wear my bridgedales in my Dunlop purofort thermo+
Some pairs slip down, others don't.
Neoprene boot socks with a pair of regular cotton socks inside. It’s what I’ve worn in my Muckboots for years.
Mine are ankle height, sort of bootee things I got from a country store, for wearing in wellies, and I have seen a similar thing for wearing inside work boots, but I’m drawing a blank finding anything like them at the moment. Something like these for surfing and kayaking would probably work just the same, with thin sports socks inside.
https://www.nookie.co.uk/product/ti-liner-2mm-neoprene-socks/
I believe the practice is to fold the tops of your socks over the tops of your wellies. This prevents them from slipping down and bunching up around the toes.
The big fashion statement you have to worry about is whether its good form to wear your trousers tucked in or over the top.
Seriously, get some Meindl Jagdsocks (Hunting socks)
Merino mix, excellent fit to calves, knee height, keep their shape and stay up. I've worn nothing else with wellies or walking boots since I got my first pair back in 2018.
https://beckettsstore.com/products/miendl-jagd-socks?variant=37116393816217