The fact that they know nothing is shown by the fact that everything we know about Socrates (8th) comes to us from Plato's (4th) Dialogues!! I'd say Marx should be #1 if it comes to the most HARMFUL influential philosopher. None could rival his influence for its destructive and demonic results.
I would argue it was Jean Jacques Rousseau, who is the spiritual forefather of Marxism, and also the favourite philosopher of the French Revolution.
Marx was an overly verbose idiot. The Labour Theory of Value is the biggest piece of economic stupidity ever suggested.
Regards, Ivan