Any server with a population of 'High' or 'Very High' is very lively.
Also note that your characters are not locked onto a server in the same way they are in World of Warcraft. You select a 'Home World' which is going to be your default server and the server you will represent in WvW, but you can 'Guest' indefinitely on up to two additional servers per 24 hours. You also have the option to change your home world. Guilds are also not tied to servers.
Essentially you have a lot of freedom to go where you want at any given time.
I'm on Far Shiverpeaks.