Voted by 531 philosophers:

1. Plato (Condorcet winner: wins contests with all other choices)
2. Aristotle loses to Plato by 231–229
3. Socrates loses to Plato by 366–81, loses to Aristotle by 356–122
4. Aquinas loses to Plato by 447–37, loses to Socrates by 328–130
5. Augustine loses to Plato by 458–22, loses to Aquinas by 279–115
6. Epicurus loses to Plato by 458–15, loses to Augustine by 291–106
7. Parmenides loses to Plato by 453–16, loses to Epicurus by 170–162
8. Heraclitus loses to Plato by 447–21, loses to Paremenides by 179–114
9. Confucius loses to Plato by 417–20, loses to Heraclitus by 152–150
10. Ockham loses to Plato by 462–9, loses to Confucius by 158–156
11. Anselm loses to Plato by 452–10, loses to Ockham by 162–150
12. Pythagoras loses to Plato by 457–12, loses to Anselm by 161–152
13. Duns Scotus loses to Plato by 436–11, loses to Pythagoras by 151–145
14. Machiavelli loses to Plato by 459–9, loses to Duns Scotus by 156–153
15. Democritus loses to Plato by 453–13, loses to Machiavelli by 166–151
16. Zeno of Elea loses to Plato by 458–10, loses to Machiavelli by 172–146
17. Plotinus loses to Plato by 441–7, loses to Zeno by 154–118
18. Avicenna loses to Plato by 426–8, loses to Plotinus by 134–114
19. Cicero loses to Plato by 450–6, loses to Avicenna by 140–134
20. Sextus Empiricus loses to Plato by 437–8, loses to Cicero by 142–134
I’m surprised that the Buddha wasn’t on this list.  I would’ve made him more influential than Ockham.

On this list, I haven’t read 13, 17, 18, 19, and 20.  Looks like I’ve got some catching up to do.