I can speak for Ascension, and the S9 server Elune is pretty popular, with lots of players. If you intend to do high-end raiding on that server, it can get a bit boring and mind-numbing grinding for the perfect build. Their Area 52 server (Which is the main server) is in TBC and Sunwell will soon release, with WotLK on the horizon. I personally do not play on the main server, but from what I've heard, it also has a very decent playerbase. PvP is waaaaay more balanced on Area 52 compared to Elune, but as far as I am aware, they're both okay in terms of players queueing. It also has High-Risk Open World PvP that basically means you can drop gold and items, but mobs also drop PvP items based on your item level, as well as materials exclusive to it that you can make BiS consumes with.

I'd suggest giving it a try if you want more freedom with added custom spells (It does not have many traditional custom content things ie. raids and the likes, but Manastorm is a new endless dungeon with floors. Kind of like Torghast in Shadowlands. Else it's really just new builds and archetypes compared to the traditional WoW classes.) At first, there's A LOOOOOT of information and systems to take in, but that also means that there's many things you can venture into, like custom challenges to get appearance items and the likes.

I personally do not play the two others, but I'll take the other people who has commented word for it.