I'd personally have to go with invisibility for the usefulness factor.

As one of the resident comic geeks and advocates here on Steemit, I just have to recommend Astro City #1 written by Kurt Busiek. It's a fascinating take on a Superman-like hero known as Samaritan. During the course of the issue he flies about the globe, racing at hypersonic speeds, pushing himself to rescue and aid as many people as possible.
At days end he collapses into bed, exhausted. As he drifts off to sleep he tally's the days flight time in his head, "Fifty-six seconds, best day since March." As he sleeps, he dreams of flying.

While invisibility seems a power ready to be abused, it's interesting to think about how you would use any power, and to what extent your sense of responsibility (or lack thereof) would consume you.

Incidentally this comic is a free download on a lot of digital platforms such as iBooks and Amazon's Comixology. Yesterday was Free Comic Book Day, but if you missed it, indulge in some heroic power fantasizing today!