Some of you might disagree with this pick because Reaper has to be close to his enemies to be effective, and getting closer to your enemies increases your chances of being killed. But keep in mind that the closer the Reaper is to his enemies, the harder his guns hit.
And boy, do his guns hit. If things start to get hairy, just use the 'shift' ability and run away. Easy, right? Reaper is one of those heroes who's easy to play but quite hard to master, like all the other characters of Overwatch.