Oh god, please don't get me started on Deadliest Warrior. Its like 300: I absolutly LOVE it, but if that is where you get all your ideas on history then you have some serious issues.

I would say the Romans were certainly one of the most successful. They had the largest empire in history. (You could say that Ghengis Khan or Alexander the Great had a larger empire, but those only lasted as long as that person lived. So those don't count.) The only enemies they couldn't defeat were the Britans and some of the northern Germans, at least until the Hun's started their little rampage accross Asia. They had the best military of their time and the best technology.