Best quote from the whole series?
There are very many to choose from, but I reckon it's:
HACKER: "You mean to seriously tell me that winking at corruption is government policy?"
SIR HUMPHREY: "No, no Minister! It could never be government policy, that is unthinkable! Only government practice."
Incidentally, for my money, the best from Yes Prime Minister is where Humphrey has been cleared of being a Soviet spy, and Hacker addresses him:
HACKER: "Ah! Goofey... er, Humphrey!"