UFC obviously has the most depth, and is the most successful. There's no debate. As a long time supporter of the UFC, I just hate where they're going and what they've been doing. Everything went south starting with the FOX deal.
I get the most excited for Bellator events these days. I can't really explain why, I just do. I just like they're format more and you really don't have to invest much into it, except watching/DVR the event on Friday nights. I still watch the UFC, and I realize they're still the #1 org, and probably will be for some time. But as far as my favortie goes, Bellator is #1 right now.