The short answer? Good PR.

The long answer?

Albert Speer was called a "good Nazi" for two reasons:

Firstly, he was one of several Nazis to show remorse during his trial before the IMT in Nuremberg.

Secondly, he was able to convince the tribunal that his crimes were relatively minor compared to other Nazis. To give an example, Hans Frank also expressed remorse and converted to Catholicism, but he had been Governor-General of Poland, so he was never going to wriggle off the hook.

His claim at Nuremberg was that he was very sorry for committing some crimes, and that he hadn't been aware of the Nazi regime's "crime of crimes", the extermination of the Jews. Unfortunately for him, these claims were revealed to be false by Matthias Schmidt, Gitta Sereny, and by correspondence unearthed in 2007.

To whit, Speer was present when Himmler discussed murdering Jews.

Speer used the Nuremberg Laws to evict Jewish tenants to make way for German families, sending the Jews to be exterminated, then rewrote the Department for Resettlement's Chronicle to remove references to this.

Speer, as de facto head of production, must have been aware of the vast use of brutal slave labor to bolster production.

At the trials, he used his charisma and his supposed remorse to evade the death penalty, and in later years his status as a surviving senior "Nazi who said sorry" and his books let him become rather rich and liked. Trouble is, he was neck deep in the Nazi crimes.

Good books:

Tooze, Adam Wages of Destruction: The Making and Breaking of te Nazi Economy

Sereny, Gitta Albert Speer: His Battle with Truth

Schmidt, Matthias Albert Speer: The End of a Myth