You are possibly referring to this:
After visiting these two places you can easily see how that within a
few years Hitler will emerge from the hatred that surrounds him now as
one of the most significant figures who ever lived. He had boundless
ambition for his country, which rendered him a menace to the peace of
the world, but he had a mystery about him in the way that he lived and
in the manner of his death that will live and grow after him. He had
in him the stuff of which legends are made.
This is from Prelude to Leadership (The Post-War diary of John F. Kennedy). See page 73 to 74.
But what you quoted is mixed with this:
As to his popularity, especially among the youth of Germany, there can
be no manner of doubt. The old trust him; the young idolise him. It is
not the admiration accorded to a popular Leader. It is the worship of
a national hero who has saved his country from utter despondency and
David Lloyd George: Daily Express, September 17, 1936
Also see this.