The best empire for dominating victories is America, hands down. You get a feel for what it takes and just go from there. After the middle of the game, America really cuts lose. It is slow at first though. The home fefence gives combat strenght in the home continent and then there's a fend off.
The legacy allows for granting government bonuses too. When the modern area is finally reached, you'll have unique military units which is a plus because those units can defeat any contemporary, no problem. America is by far the best empire to dominating victories.
There is no terrain and it's versitile too. Some prefer tother empires but to me, there is not a question about it, America is the best one if you want to be granted territories twice as quick and...who doesn't?