23
Depends on how we're determining "best". If it's conquest ability, then the Romans as the Americans don't conquer. If it's win / lose, then America has technically never lost a war (now before everyone starts saying "But Vietnam!!!", for America, that was a Police Action... not a War. Congress never issued a State of War).
This is at least to my knowledge at 0600 (GMT -6) before I go to Biochem. I'm having trouble focusing on U.S. history right now.