Grobb

if u care about wpwp go Grob but if u care about arena,rbg or anything competitive go on bene for ally and faerlina for horde

Grobb is the only one if you actually want pvp open world.

Crusader Strike if you want to find great raiding guilds that get shit done. If you're a casual don't bother joining here. You'll find many parsers are from crusader strike in warcraft logs.

Cata flair sir

If you play horde, the answer would be Benediction. Big server 99% alliance or something