Most noteworthy would be Hess' flight to England in 1941, although that was not quite a defection.
history.stackexchange.com/users/3116/Oldcat
Defection rather implies changing sides in a conflict.
None of the senior Nazis seem to have done that:
Hess seems to have wanted to persuade the UK to make peace with Germany, but his idea of how to do this was hopelessly wrong.
He seems to have assumed that the UK monarchy of the 1940s worked like the pre-WWI German Empire.
Himler was trying to preserve his existing position as head of the SS and Police, and add the leadership of Nazi Germany to it, by getting the Americans, British and French to join the German side in the war against the USSR.
This was considered treason by the existing Nazi government, but was not a defection to the Allied side.
Surrendering yourself, or the forces under your command is not defection.
If German forces in Italy had switched sides and made war against Germany, that would be defection, but they didn't do that, and the Allied side wouldn't have been interested anyway.
Willy Lehmann was a spy, but spies aren't usually considered defectors unless they visibly change sides, or flee to the side they've been working for.
Paulus might be considered a defector, at the point when it was starting to become clear that the Germans weren't going to win, but he was a military officer rather than a politician.
Ernst Hanfstaengl quit the cause before the outbreak of war, but it's hard to call him a senior Nazi.
history.stackexchange.com/users/19407/John Dallman
Operation Sunrise led to the surrender of all Axis forces in Italy on 2 May 1945, 5 days before VE day.
Several high ranking officers were involved in the negotiations and the story is complicated, but Field Marshall Albert Kesselring, the Commander in Chief of the Mediterranean Theater, knew of the negotiations and ultimately approved - I would call this a 'noteworthy defection.'
Allen Dulles' book 'The Secret Surrender' tells the story.
history.stackexchange.com/users/1172/stopple
In April 1945, toward the end of the Nazi regime, Heinrich Himmler , the head of the Gestapo (Secret Police) and "SS" military units, left Berlin against Hitler's orders, and began negotiations with representatives of neutral Sweden, as an attempt to surrender Germany to only Britain and the United States, while allying with them against the Soviet Union.
He did this on his own authority, without the consent of Hitler (who expelled him from the Nazi party), in fact, against Hitler's express instructions to stay in Berlin.
His overtures were rejected, but he was basically the highest-ranking Nazi (fourth after Hitler, Goering and Goebbels) to try to negotiate with the Allies without authority from Hitler.
(Admiral Doenitz, who did surrender to the Allies, did so with authority vested in him by Hitler's last will and testament.)
history.stackexchange.com/users/120/Tom Au
I'm not sure whether being captured blunts the definition of "defection" at all, but after Field Marshal Friedrich Paulus' capture, he cooperated with the Soviets completely, even making anti-war propaganda statements for the Soviet Union .
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How about (for an EARLY example) Fritz Thyssen ?
He was a wealthy industrialist who gave large contributions to the Nazis from 1923 to 1932.
He was one of the major characters who urged von Hindenburg to appoint Hitler chancellor and was elected as a member of the Reichstag.
However he soured on the Nazis (and Germany) at the beginning of WWII and took his family to Switzerland (and then France), after which he was expelled from the party and his company was seized.
Later he made the mistake of going to Belgium right as the Germans invaded and he ended up in a concentration camp, but managed to survive the war.
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Try to consider Willy Lehmann.
He was recruited in 1929, before Hitler came to power, initially for money, but when the War became expected to start, his collaboration became more ideological as he was strongly against it.
He headed the Gestapo department that was doing counter-intelligence against Soviet industrial espionage.
He transferred over a lot of very important info to the Soviets, that on German weapons, rocket technology, submarines, Gestapo cryptographic codes, and even the exact date for the invasion of the USSR.
When the USSR had lost link to him, he conducted a risky step, and in 1940 put a letter into a post box of Soviet embassy calling for continued collaboration and said his work in Gestapo would lose any meaning if the USSR would not resume contacts.
In 1941 after invasion of the USSR the connection with him was lost.
In 1942 the Soviets tried to reestablish contact, but the Germans captured the sent communicators and executed Lehmann afterwards.
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During WWII, before it became clear the Nazis were going to lose, is there any especially noteworthy examples of a member or members of the Nazi party defecting to the Allies?  I'm aware  noteworthy  is fairly ambiguous, but I'm not really sure how else to phrase the question.
What is the most noteworthy defection of a member of the Nazi party to the Allies during WWII?
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None of the senior seem to have defected. Himmler did leave Berlin against orders at the end of the war. No senior Nazis appear to have done this, however, depending on the definition of 'defection', some might fit the description, including Hess, Himmler and Hanfstaengl. There were also a number of captured Nazis who went on to work for their captors. Willy Lehmann might also be an acceptable answer.
No senior Nazis appear to have done this. There are number of potential candidates to be described as 'defectors', including Hess, Himmler and Hanfstaengl. If capture is relevant, Paulus and other captured Axis forces would be relevant. Another potential candidate is Willy Lehmann.