The ability to manipulate time at will.
I could use the ability to quickly amass large amounts of wealth, and although that might seem selfish, in reality, money is power, and the more money you have, the more control you have over the world, take politics for example, the mostly likely to win presidential candidate is usually the one with the most financial backing/support.
If you were the richest man in the world, you could easily change things for the better, or for the worse if you wanted.
whole countries can be and have been bought and sold to the highest bidder.
heck, you can every buy up every corporation, and change their polices it ways that aren't so derogative to society.
like for example, buying a major record label and requiring them to only play clean music, with none of this heavy metal or gangster rap stuff anymore.
you could even take over corporations with the soul purpose of running them into the ground, like with cigarette manufactures.
not to mention spreading the wealth to the poor, feeding the hungry, rebuilding dilapidated cities, funding initiatives that would help mankind and so on.
It's not just money though, with the ability to control time, you can prevent disasters.
it would also help a lot in keeping you alive when others with power who feel threatened by you and decide to have you killed.
just keep the slow mode on when out in the open, and you can see any sniper bullets flying your way, any cars trying to run you off the road or rewind time once your plane 'accidentally' starts going down, so you can chose not to take it.
and of course it would help predict your opponents moves who try to fight back in less direct ways, like trying to frame you for crimes.
gotta stay alive to see the project through to the end after all.