Gandara
Veteran
As far as I'm concerned, server only matters for guilds and auction house purposes, everything else is cross realm. When it comes to twinking though, there are a couple popular servers, such as Aerie Peak and Bleeding Hollow. Kel'Thuzad also has some twinking action on the Alliance side, mainly with the guild Teldrassil Firefighters. There are probably other popular twinking servers as well, but I don't know a lot about them. Maybe someone more knowledgeable on the subject can provide a more concrete answer to you.
Sprite
21s Coordinator
As a f2p the only thing you’re going to have to worry about in relation to your server will be finding people out and about in the world, especially with all the different chromie time phases. As mentioned above guilds and auctions will be something you’d be missing out when playing on a weird server like idk ghostlands or something with zero pop, but being f2p you don’t have access to that stuff anyways.