I say Pathfinder.
Why? Because I asked several people with no interest in astronomy your question (adding Pathfinder) and Pathfinder was the only one anyone had heard of.
Public interest does more for NASA budgets than most anything else, and that one mission single handedly did so much for interest in exploration.
Others may learn more, but if the public isn't in it, it won't help NASA get more funding for more missions.