Taking down Reaper before Wraith Form
As of the new patch (1.7.0.2) on January 24, 2017, Roadhog's hook mechanics have been changed. Any enemy that has been hooked by Roadhog will now be pulled closer than in previous patches, which allows Roadhog to more reliably one-shot them. This includes heroes like Reaper and Mei. As a result, this answer isn't really accurate anymore; however I will keep it here for documentation's sake.
Due to Reaper's higher starting health (250), and the fact that his wraith form has no channel time, killing him can be very difficult if he's starting at full health. Add to the fact that Reaper can actually queue up his wraith form while being hooked by holding the button down, it can be near impossible. (The same idea goes for Mei, as well as other champions who always seem to live after getting hooked (Ana, for example)).
After reading through this Reddit thread, it turns out there is a way to kill Reaper more reliably after hooking him with Roadhog. After successfully landing a hook, turn and look about 90 degrees away from where you shot the hook in the first place. For some reason, this causes the hooked enemy to land much closer to you, which means that more bullets will hit them when you shoot them. People in the Reddit thread have reported being able to oneshot a Reaper much more consistently using this strategy.
Another simpler approach when dealing with Reaper or Mei is to shoot them from a distance (preferably with your alt-fire), then use your normal hook combo to finish them off. (Or only grab them if you know they've taken some damage first.)
One final way of dealing with Reaper is highly dependent on Reaper's reaction time and his latency. After hooking him, take a short step forward, jump, and shoot downward toward his midsection. This will cause some of the pellets to be headshots, dealing extra damage. Finish off with a melee attack, and he should be dead. The problem with this approach, of course, is that if the Reaper does not have high ping, you've just given him an extra quarter second(-ish) to activate his wraith form, possibly negating all the damage entirely. This strategy should not be used often since it is so unreliable, but I wanted to include it to be thorough.