Season 5, without a doubt.
Season 5 had everything you could ever want from the show... two coaches feuding, the house filled with complete nutjobs, and AWESOME fights. Plus, I've kept it no secrete... I've always been a huge fan of BJ Penn and Nick Diaz, so with BJ as a coach and Nick's little brother as the star of the show... it was the perfect season for a fan such as myself.
But yeah... so many great moments that season. BJ's raise your hand picking process, Cory Hill's barking and talking to himself, Gabe not being able to cut the weight, Andy Wang's crying, The Marine and cocky guy fighting in the yard, Joe Lauzon being the awesome dude he is, and Nate Diaz nearly coming to blows with Karo Parisyan... the show had everything you could ask for and more. Great season. The only thing that could have made it better is if Lauzon hadn't got lay 'n prayed on by Manny Gamburyan, since that would have given us a Nate Diaz vs. Joe Lauzon finale, which would have been one epic fucking fight.