Here's an at-a-glance guide to who's finished where from all the previous contests, and for the sake of getting meta I've added a ranking system. 1st place gets five points, 2nd gets four, 3rd gets three, 4th gets two and 5th place gets one point. Of course, this system heavily favours users who have entered many competitions, so in the interest of fairness, as of July 16th 2015 the list also shows a comparison of how many contests each user has entered, along with their average-points-per-contest score. The three highest averages under this system are marked in gold, silver and bronze respectively.
So, here's the leaderboard as it stands after the What Happened in 2015? contest:
...And below are all the old ones, for posterity.