All one-player mounts are the same default speed. Only two-player mounts can have different speeds, but none of the two-player mounts are any faster than the default speed (as far as I know), only slower (ex. the turtle mount).
So at the moment, any one-player mount and most two-player mounts are the fastest mount. You'll definitely get the fastest with any one-player, however. (which is default speed so...)