On a true singlespeed, you should have both, as you'd have no (good) way to stop should the one go out. If, however, you're on a fixed gear, and can therefore apply back pressure as a brake - or if you really want to just have one, then the brake should be on the front.

There's nothing about having a rear brake (other than the additional, extremely minimal weight of the cable and calipers behind the center of gravity) that'll keep you from "fly[ing] off" under maximum braking, as the rear wheel will already be completely unweighted and any brake back there won't be at all effective. Now, under certain conditions, e.g. slippery roads, rear braking is safer than front braking, but it's never more effective.