The Reelz Channel has a good documentary on their show, "Murder Made Me Famous", that seems to come to the conclusion that it was Aaron Kosminski.
But The History Channel has another good documentary on their show, "MysteryQuest", that points the finger at a guy named Francis Tumblety.
They compare his handwriting to that from the "From Hell" letter that was sent to the police. The "From Hell" letter is the only letter that Scotland Yard believed was actually written by the killer.
The results of the comparison are pretty compelling.
Good stuff.