Argent Dawn appears to be in the worst shape, as Blizzard is sending players from that realm onto three different realms. Magtheridon seems to be the least busy -- all they're transferring off there is the Horde-side players. They're also transferring players off of a few different realms to the newly-created one, Chamber of Aspects, so if you're looking for a new start, there you go.
These migrations are going until December 2nd, so now's your chance, EU players, if you're on the selected realms and have been waiting to jump ship.
Al'Akir => Dentarg, Xavius
Argent Dawn => Darkmoon Faire, Steamwheedle Cartel, The Sha'tar
Burning Legion => Ahn'Qiraj, Daggerspine
Crushridge => Bladefist, Karazhan
Drak'thul => Burning Blade, Deathwing
Grim Batol => Dentarg, Xavius
Magtheridon (Horde only) => Zenedar, Haomarush
Neptulon => Ahn'Qiraj, Daggerspine
Outland => Bladefist, Karazhan
Ravencrest => Burning Blade, Deathwing
Sylvanas => Auchindoun, Lightning's Blade
Twilight's Hammer => Auchindoun, Lightning's Blade