That’s a tough one, because there are so many. Off the top of my head, runners-up would certainly be:
- David—an obvious choice, as the man with so many sides to his character
- Solomon, his son—because I’d like to see how wise he really was.
- Hezekiah—because he ordered and oversaw the fusion of the Judean and the Israelite (northern) religions into one, after the fall of the northern kingdom
- Samson—because it would confirm whether his Dan tribe was, as many historians suspect, none other than the Grecian Danites who were related to the Philistines but chose to throw in their lot with the Israelites
- Jacob—because he starts out the weakling but sly brother of Esau, but somehow evolves into a man capable of wrestling an angel of God all night.
But in sheer intriguedness, it would have to be Isaac:
- Because he never speaks to his father again after the Binding (which is not surprising, when you think of it), and
- Because he may possibly be completely unrelated to Abraham, because the story of Abraham being his father may be a fictionalised product of the unification of the Judean and Israelite traditions.
(Originally written in reply to a question at Quora.com)