UG, followed by Sherdog and I have no clue whose number three, it might be this site but I couldn't tell you, maybe it's the UFC.com forums.
You'd think this would be the biggest since MMA Forum is probably more googled than Sherdog or The Underground.
2013 VIP FFL Champion
THE RETURN - 9/27/14
Machida - Alves - Cruz - JDS - Weidman - Condit - Pettis - Edgar - Cowboy - Dodson - Okami