I seem to remember that George Tenet was bill clinton’s pet weasel. Clinton installed him as head of the CIA, and clinton had a record of never appointing anyone but loyal stooges to important posts like that. Maybe clinton’s one mistake was Louis Freeh, and most likely he only screwed up with that one because he was in a big hurry to dump the sitting director and didn’t have much time to look around.
George Bush’s mistake—and it was a whopper!—was to leave Tenet in office. Maybe it was the single biggest mistake he made.
I'd say having Powell as SecState was at least as bad.
His biggest collective mistake was to leave any member of the previous administration in their posts.