Some illustrious names mentioned, including Napoleon Bonaparte..but when the votes were finally counted..the toughest yet still "worthy" opponent was selected.. The choice; George Washington..
I would like to say their "Gentleman Johnny" Burgoyne was our toughest and most worthy...and he was very much that way.
However, I must tip my hat to Gen Robert E Lee. The toughest foe the USA ever fought was essentially...ourselves..
http://www.telegraph.co.uk/history/9204961/George-Washington-named-Britains-greatest-ever-foe.html