He was in charge of the foreign ministry when some interesting (wouldn't say amazing) accomplishments were made (Naval Conference, Aunchluss, Munich). But he was not the architect of these policies. The London Naval conference was Von Neurath's Idea, and its only a miracle that Ribbentrop didn't blow that. He delivered an ultimatem to the british and the very beggining of the conference saying that Germany would be allowed a navy 3/5s the size of the Royal Navy or the talks were off. The British were more surprised then anything else and asked him why he was doing this, he responded that he read somewhere that it is important to set out guidelines for negotiations.
Then we have the Aunchluss, which was organized largely by Goring, so Ribbentrop again gets no credit.
Then we have the Munich agreement, which was hammered out by Hitler himself with Mussolini and Chamberlain. Ribbentrop actually was distrustfull of Munich because he thought it was some sort of convaluted trap by the British. He advised Hitler to go to war IIRC rather then negotiate.
Then the fall of 1939 roles around and you have his single usefull accomplishment, The Molotov-Ribbentrop pact. Though considering that Stalin had been pushing for a pact for years, it seems something easily within the reach of the more skilled Neurath or just about anyone else that Hitler could have appointed.
Then we have Ribbentrop's most notable diplomatic accomplishment. Advising Hitler that the British would never, ever go to war over Poland, and the war would remain localized. We know how well that went.