I never think of Temple as a prequel. I suppose by definition it is considered one because it happens before Raiders. However, it's completely unrelated and could take place just about anywhere on the time line. It just happens to have 1935 after the titles.
In fact, that's one of the things I like about it. It doesn't rely on the other films or characters. It's a stand-alone & complete adventure.
As far as good prequels are concerned - I think Godfather II is the gold standard.