I don't really know which one could be loaded to be the most flat...but I would suspect it would be the 6.5 X 55,
And I don't know which one could be proven to be the most accurate...but I suspect the 6.5 X 55 would be in the top 2....it has been winning european and world shooting competitions (along with other calibers) for years.
If you don't mind the longer receiver then I'd pick the 6.5 X 55. But it would be hard to argue with picking the 308 b/c of the ammo availability............ACTUALLY, IT WOULD BE HARD TO ARGUE WITH CHOOSING ANY OF THE CALIBERS YOU MENTIONED...I have a 308 and several 6.5 X 55 but would love to own a 7mm/08 and like the idea of the 260 (almost as much as i like the 6.5 X 55!)