My friend, you test me, as you are unequivocally more qualified on this matter, but supposing that this is more a riddle than a question, the “best” submission from the mount is the one your opponent gives you. I would also contend that any submission that does not compromise your position in the attempt is superior to one that does. This being said, a standard arm bar is less ideal than a belly down arm bar or choke.
My personal favorite is the triangle choke. By allowing an opening I encourage an attempt to escape out the back. This affords me several options including the opportunity to maintain a dominant position if I should miss the attempt. Here is an example of the fine gentlemen at Submissions 101 demonstrating the technique.
Bear in mind, this video shows falling to the back to complete the submission which is not necessary and compromises your position. I suggest finishing it from the top.
Now that I have made my case, I would like to know your thoughts.

August 19th, 2009 at 7:26 pm
The picture is sweet. How about adding a video of a triangle choke for the readers that aren’t in the know.
August 19th, 2009 at 8:53 pm
Roger Gracie cross choked something like 4 dudes in the 2009 Mundials from the mount……Mount is a tricky position I think. I guess I would say the best sub from there is the one you can pull off the best, but that is a libby bleeding heart answer. Mounted Guillotines.
August 21st, 2009 at 10:19 pm
Ask and ye shall receive Mr. Highland.
September 8th, 2009 at 6:05 pm
Cross collar choke, for sure. Initial collar grip underhand with followed cross grip overhand and gripping to the shoulder of the gi. My personal favorite, at least. This attack allows for a setup to “S” mount and the armbar or if you are reversed you can maintain your grip and finish the choke from guard.
September 8th, 2009 at 7:02 pm
I really do like this choke. Luciano Olivera pulled that little number on me, but in committing his post, I was able to sweep him. Sweeping a black belt is not a common occurrence for me. I have a sneaking suspicion that he allowed the sweep because he already had his grips secure, as you mentioned. I was saved by the bell in that particular instance, but I definitely see how effective it can be.