Really just comes down to how you define "best." Some people prefer a larger base to reach out to, some people prefer a smaller group of familiar faces. If you'd like to do arenas, though, AP is the only US Horde server I'm aware of that does them consistently (or at least, they do them with the most frequency).
With that being said, I'm going to assume that by "best" you mean largest active player base, in which case I'm pretty sure AP would be the clear winner.
And as Salfir said above, Yde's post would be a good reference point, too.
Edit: Guess I was writing this while you were posting that you're EU. Don't have much knowledge of the EU servers, but I think Aggramar is probably your best bet. Damn, Salfir said everything I was gonna say...