Just as a guide, the current situation is like this:
Major four clans:
The WolfPack (Autumnwolf17)Vortex Warriors Clan (BeatlesFan)The Killer Mantids (GivenUDaHimelik)The Military Soldiers (Ryankill)Others
Warrior's Creed (Vortex_Master)The Order of Secrets (Unknown)The 4 Riders (Ozserf)The Wolves and the Vortex Warriors are the biggest two by far; that's my clan and BF's. The Mantids and Soldiers are next, though there are a few downsides to those (younger players and difficulty for new members, no offense to either)
I would also recommend WC, they are the oldest clan on the list and were the best for a long time. They have been going through some inactivity but could easily rise back to the top with a few committed members. They have really excellent leadership.
As for the last two, they really aren't options for you as OOS is an anonymous clan (they use fake accounts and all) and 4 Riders is an exclusively high level clan.