Trabia and Wutai seem to have the highest NA population by my guesstimation.

Trabia also has the bonus of rhyming with a part of the female anatomy.

The time you're playing also has a bearing on whether or not there will be a higher concentration of English-speaking people. Wutai is pretty active in the evenings Eastern time.

Besaid is the unofficial "role-play" server, so you will very likely find lotsa people there, too.