I really like this list, except I just don't see how ME2 and Splinter belong there. I would have also added Just Cause 2, and I heard Assassin's Creed 2 has a great open world as well, but heck, it's your list.
My vote goes to Red Dead Redemption. Other games look terrific, but it seems quite a bit easier to create the symmetrical geometry used for the areas, buildings, etc. Creating a wide, flowing, natural looking landscape is a bigger challenge, and from what I'm seeing RDR is shaping up to be the best of the Old West genre. Last one I played was Gun and well, it was pretty weak.