In terms of being able to influence the largest amount of people, I'd say POTUS. The president could give a speech about debt forgiveness and crash every stock market in the world, if he so chose. And that doesn't even touch on the amount of military power under his direct control, which he can deploy anywhere in the world at any time, for whatever reason, and it is perfectly legal to do so.
In terms of realistically getting things done without being held accountable, I'm thinking Putin. That guy can literally do anything he wants. If he wanted to eat babies, he could do it, and nobody would say a dammed thing. And even though Russia is somewhat diminished in terms of military might, there is still only one country in the world that could stop them if they decided to take over Europe.
China is getting there, but their position is too shaky for them to start to force hands. And Russia can still kick their asses, and the US would dom them so bad it's laughable.