According to Mr. Scott, the best diplomat is a fully-armed phaser bank.
Seconded…was the one I thought of when I read the thread title.
The Byzantine emperor Alexius Commenus has be one of the most impressive examples of Diplomacy in history. Successfully kept his empire alive by skillfully playing on enemy off against another, and skillfully managed to hussle the First Crusade through Byzantine territory without it descending into chaos. How good a job he did is witnessed by how soon it all went to crap with less competent emperor on the throne.