Striking battles and knockouts always please the fans, but the true fans of mixed martial arts can appreciate an outstanding ground battle and a ridiculous submission.
A fighter can be on the brink of defeat, slap on a submission, and force his opponent to make a decision.
Go to sleep, or admit defeat.
Here is someone in every weight class who is most likely to force that choice.
(I'm aware that an arm bar wouldn't put somebody to sleep, but it sounded cooler than saying go to sleep or lose a limb)