Unfotunately, this fight will probably never happen. Aoki says he isn't going to the UFC and Penn is currently under contract with the UFC. This will end up being one of those fights that finally happens 5 years down the road and everyone will say it should have happened five years ago. As skilled as Aoki is - I think Penn mauls him brutally.
[YT]<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/TqXwfEaK89A&hl=en&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/TqXwfEaK89A&hl=en&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>[/YT]
Most of us would agree that BJ is the better fighter (even Aoki admits it!) but having Aoki at 1 is debatable. It all depends on the persons own opinions so I can kind of understand why they have him at 1. What does the person doing the rankings value more? For example most recent wins, look at the fighters skills or do you just have the fighter who you think is the best regadless of who he has faced? Even though he lost to Hansen, he has been more active and has had a very good year. He beat JZ, Alvarez (who both had very high rankings when he beat them) and even Uno who was coming off a victory over Ishida. Rankings in general can be tricky and having fighters spread across different orgs doesn't help. The best way to clear things up for certain is through a fight. Unfortunately this is one fight which isn't going to happen anytime soon.
lol BJs ground game is better than Aoki..His pure JJ is a couple of levels above as well. Im not sure if Aoki has faced guys who can really defend subs.
If we're talking submissions and overall ground skills...(meaning not necesarilly Jiujitsu) they're more or less on the same level. Only difference is that Penn seems to have superior wrestling combined with a superior g&p offensive. Aoki's submission palette is already one of the most varied in the sport. Having said all that... i think Penn would find a way to KO (or TKO) aoki. But Aoki's chances of winning wouldn't be very thin imo.
