In a UFC that is constantly raising the bar for exciting fights, I've come to find watching Jose Aldo dismantle opponents to be among the most entertaining.
I also really enjoy watching Sergei Kharitonov, Gilbert Melendez, Shogun, Nick Diaz, Mamed Khalidov, Daniel Cormier, Fedor, Overeem, and Anthony Pettis. I know I'm forgetting a bunch on top of those