Depends on your skill/competitiveness.
Alliance - Never Got My Glaives is a more casual (compared to others) guild. 1800 rating is required. I've been there, everyone is nice and always pvp'ing.
Alliance - Hero is the best Alliance twink guild. Require 2.2k at least in arena. Don't get on Qpac's bad side.
Horde - Retro is good, haven't seen them around in a while. I heard the server is going dead, not even sure of their requirements.
Horde - Nerd Cleave is by far the best. Require 2.2k to get in, everybody knows what they're doing.
Horde - Static Gaming is a spin off of nerd cleave. They also require 2.2k and are equally as talented.