Dangerous can mean different things, I suppose. But when I think about it in this context, I think about fighters who stand the biggest chance of doing two things: harming their opponent and wanting to harm their opponent. Someone for whom, as B.J. Penn has said, fighting is not a sport, but a fight. Someone you can see relaxing after the fight with a trank dart or two.
Records and statistics are certainly part of the equation, but recognizing someone as dangerous largely happens on a gut level. Do they fight with bad intentions? Are they dirty? Are they, eh, temperamental? The best or most effective fighters aren't necessarily the most dangerous.
Here are my top 25 examples, covering alllll of MMA history. With apologies to "Lightning" Lee Murray, this list only considers actions inside the cage. Did I miss one? Let me know in the comments. But hey, let's try to contain our aggression out there.