Penn because of how skilled he was. He had everything you want in a fighter from a skills standpoint.
-Elite boxing with underrated kicks/knees. The flying knee he nailed Sherk with and the high kick on Diego were just beautiful.
-Elite and creative BJJ. The liver kicks from back control to set up the choke on Florian and the jailbreak on GSP spring to mind.
-Great finishing ability (the only other fighter in the lower weight classes [155 and under] who could hold a candle to Penn when it came to finishing elite opponents is Aldo). Excellent blend of KO power, submission skills, and killer instinct.
-Great takedown defense.
-Very good takedowns (name 5 LW's that could take down Fitch and secure back control. I'll wait.)
-Amazing durability - he has a granite chin and prior to the Diaz and Rory fights (in a weight division he had no business competing in anymore) I don't think I had ever seen him cut.
The lack of a real strength and conditioning program (minus his time with the Marinovich's which unsurprisingly resulted in two of the best performances of his career against Florian and Sanchez), refusal to gameplan, and poor response to adversity were his downfalls. Amazingly talented fighter though.