Walter Scheel, because he was no Chancellor.
(There's no formal position of "Acting chancellor" in Germany and almost nobody in Germany would claim that Scheel was Chancellor once. He was foreign minister and vice chancellor and for a few days during his term there wasn't a Chancellor, so he was the highest-ranking government official for this period of time. If we pursue the nitpicking even further: Scheel wasn't acting chancellor, he was acting head of government. You're Chancellor if you're elected by the Bundestag. No election, no Chancellor.
)
