I was wondering who, as a diplomat (interpret that as you will) could be considered the greatest of all.
I am sure Dopers will come up with a list if folks I have never heard of (and probably should have).
My suggestion would be Prince Bismarck. Even his prediction (short version) that 20 years after his death the German Empire would be in great trouble- he was right almost to the month.
Talleyrand deserves a nod for his wheeling and dealing, particularly at the Congress of Vienna.
Metternich is up there.
Fictionally, Retief, of course. Factually… hm. I’d look at the Popes, perhaps?
Looking like that, Kissinger managed to talk his way into Jill Saint John’s bed.
I was going to say Retief, then apologize for this not being Café Society.
Hm. Of course Talleyrand was a great foreign minister, good call that. What about the men who opened up Japan?
James Madison.
In addition to others already named, Richelieu and Cavour deserve mention.
Ben Franklin, perhaps?
That would be Townsend Harris. But I think the opening of Japan was more due to historical inevitability than his diplomatic skills.
George Marshall.
Another vote for Ben Franklin, who charmed his way into the French court and secured a French alliance that was decisive in the Yorktown campaign and thus winning American independence.
This guy, who did much to keep an eye on Confederate espionage in Britain and to prevent the United Kingdom from intervening in the Civil War, deserves greater fame: http://en.wikipedia.org/wiki/Charles_Francis_Adams,_Sr.
Sugihara and Wallenberg, for using their diplomatic posts to save many lives during the Holocaust, are also praiseworthy.
Tallyrand. His machinations kept France intact against the actions of Metternich and the rest. By all rights, France should have been stripped back to the size of Rhode Island. But Tallyrand convinced the rest of Europe that France’s existence was a Good Thing, playing them off against each other beautifully.
Actually I think it was the gunboats lurking in the harbor more than talktalk…
Yes, well, that would be a bit of the “historical inevitability” I mentioned 
Though Perry and his gunboats didn’t actually achieve all that much in terms of Japanese concessions. The major treaty came later under Harris (when the harbor was, sadly, gunboat free.)
And look where that got us.
What about the man who opened up China? Richard Nixon. Say what you will about the man, but his administration gets 5 stars for changing the relationship for the better between the US and China as well as the Soviet Union (detente).
Both these powerful nations provided a lot of anxiety for Americans prior to Nixon.
Nixon had no particular diplomatic skills; Kissinger (and not even the State Department) did all of the heavy lifting with the Chinese secretly before Nixon flew into Beijing on Air Force One.
How about old Leo? With only words, may have convinced Attila the Hun to give up from sacking Rome?
Niccolò, Maffeo and Marco Polo, who gave us our first look inside China?
Squanto, an Oxford-educated Native American who calmed the fears of some really freaking paranoid Pilgrims and helped them survive the first winter?
Bill Clinton. He went to North Korea, gave them nothing but smoke up the ass and a photo-op and brought back two hot chicks held as spies against the state, and despite the Monica Lewinsky brouhaha, convinced his wife to publicly praise him for picking-up those two women. But he was so smooth about it there was nothing she could do except snap at some random rude African student the next week. Now that is diplomacy!
There’s also the original Gray Eminence: François Leclerc du Tremblay, aka Pere Joseph. Richelieu’s right-hand man.
Deng Xiaoping is up there, too.