Martin Heidegger or Ludwig Wittgenstein
i tend to say Heidegger, but my colleagues here continue to argue for Wittgenstein - but both of them are indispensable for many ideas circulated today.
'influential' is a tough category, far too broad to be meaningful tbh. The usual politician answers all focus around the geopolitics of the 30s (Hitler, Stalin) and the rise of nationalism (Mao, Ho Chi Min) or even the radical changes that came about during and around the fall of communism (Regan, Gorbachev) - but these are all historical actors in my opinion - an intimate part of the tapestry and hard to isolate - so I settle on intellectual history since their influence far exceeds someone who is blind to the spirit of history.