Follow up: how were they received by the public and/or US officials they may have met with?
Not sure about high ranking Nazi officials, but the US State Department maintains a historical website that lists all of the foreign leaders to have visited the US in an official capacity (i.e., a state visit) from 1874 onwards.
You can also search by country - which lists the first German head of state to visit the US as being Chancellor Adenauer in 1953.
In regards to the public reception of any officials, there was prior to WW2 the Bund which in 1939 held a rally at Madison Square Gardens where 20,000 people attended, but the mainstream public (with the exception of those groups actively at odds with the nazi/facist alligned groups) were probably ambivalent to the goings on in Europe.
Are you referring strictly to Officials? Or may we discuss military members as well? If the latter is true, then there were several military observation missions - official ones- sent prior to the USA's entry into WWII to observe the embryonic US Armor School during the late 30s.
A German Colonel from the Wehrmacht General Staff (the main planning body, later OKH) visited the United States on several occasions to observe US Army units in training (!), rate their equipment (!!) and even give guest lectures at the Cavalry and Armor schools (!!!). Is he the person of the most import to visit the United States? I cannot answer with any certainty, but his visits without a doubt are of a very interesting nature.
"[Oberst] Von Schell arrived in Louisville by train, dressed in civilian clothes, and was met by [LtCol] Willis D. Crittenberger, the [7th Cavalry Brigade, Mechanized] operations and training officer....welcomed Von Schell, but not before the visitor had changed into his dress uniform, which displayed the coveted red leg stripes of a staff officer and decorations, including an Iron Cross and a medal indicating four battle wounds...."
Source: Through Mobility we Conquer. Hofmann, George F. University of Kentucky Press, 2006.