Design wise, I think the strongest folder design I've seen is the Scott Cook Lochsa. Very similar to the Sebenza design, but the entire handle is one piece of milled titanium. That means there is really only one screw/bolt used in the entire assembly; at the pivot. Therefore, it can be made extremely strong just by using more metal, since there is very little to fail.

As far as the actual strongest production folding knife, I have no idea, as there have been a lot of pretty amazing designs both past and present. However, I will say that if I was looking for a hard use knife it would be a fixed blade, especially a Busse/Swamp Rat/Scrap Yard fixed blade. A folder is always a compromise of strength compared to a fixed blade.