Agree with Mobil about cyclone in general, the servers he mentioned do currently / soon will have / the strongest guilds imo.
But they aren't your only options, alliance-wise <Annex> on Earthen Ring seems to have a well played and decently active core. And a new guild on Terenas <Got a Token What Now> is attempting to form a competitive and selective lineup to match the other guilds. I'd contact Mchealer for <Annex> and Yawron/Firstaid for <GATWN>
<LMFAOALLYGOTOWNEDAGAIN> on Thunderhorn horde also has a core of great players who are currently looking to expand, supporting both Xfer's and rerollers with assistance. Contact Imtupac or Fearmonger, i'm guessing.
While it looks like <MNT> and <AYGAT> are/will be the 'giants' of the bg, be great to help cultivate the other guilds. More competition and action for everyone. Look at what happened to rampage, one guild loses a little activity - And suddenly everything is 'boring'.
Edit: Also keep in mind that for the servers Mobil mentioned, i'd assume [and hope

] that competition for the 'A-team' would be seriously high. Guessin'.
Edit2: You mentioned other brackets to 19,
afaik Reckoning has the best 29 action atm. Misery is crap for sure. 39/49 i've no clue. ^-^