Liquid Time
The ability to freeze time, on its surface, seems to be one of the greatest abilities with which a person could be bestowed. With it, you could save lives, travel the world and learn new skills in seconds, and shape the arc of society in your image. In fact, the opportunities granted to you by such an ability must surely be limited only by the bounds of your own imagination.
However, I would argue that the best thing you could do with such a gift is nothing at all. Time is the currency that gives our lives value, and by making it limitless, even for just one individual, you risk fundamentally corrupting the meaning of life for the entire human race. Yes, tragedy is, by definition terrible, but in bearing witness to it we develop the empathy and community that makes us our humanity. Yes, lost moments or missed opportunities are unfortunate, but it is the risk inherent to such events that inspires us to take chances and roll the dice in the hope that our lives may be made better for it. And yes, struggling and losing are never fun, but they are the very experiences that make us fight so hard to succeed and win.
As such, I believe that, should you be able to freeze and unfreeze time at will, you should never do so. Life is not meant to be lived in installments, but as a whole, and life's most precious qualities, from love to community to freedom, cannot be properly appreciated without the unceasing march of time to spur them on. The one exception to this rule, I believe, is that you could, in moments of real importance, pause time to simply cherish what you have been given, but even this should be done sparingly and with an understanding that even these moments must, one day, come to an end.