It depends what you mean by “favorite”. There are people who have ended up making contributions that are particularly relevant to me (e.g. Gottfried Leibniz, Kurt Godel, Alan Turing). There are people who would have been fun to meet (e.g. Ramon Llull, Galileo, Ada Lovelace, Albert Einstein). And there are people who managed to live their lives well in one way or another around their scientific accomplishments (e.g. Charles Darwin, Thomas Edison).

I would have two answers about scientific achievement.

First, the idea that it’s possible to make formal models of the world.

Second, the idea of computation, and particularly universal computation.