Ace Combat 6: Fires of Liberation was by far the last and best entry into the series. One of the few reasons I kept my 360 as long as I did. Unfortunately it was also 360 exclusive.
And yes Assault Horizon was pretty freaking terrible, you could tell just from the weak demo and the addition of the helicopter segments that were complete and utter trash and not what the Ace Combat series was about. They went from being a proper arcade air combat game that was amazing to trying to be the COD of flight games with Assault Horizon. Just one more reason COD's influence on the gaming universe as a whole needs to disappear.