Raptr is a service for PC gamers to adjust graphics settings, earn loyalty rewards, and "powers" AMD's Gaming Evolved app, which adds driver updating and Twitch streaming to the previous list of features. It has a sizable user base, tens of millions internationally, which allows them to rank PC games by popularity. While it is not a perfect sample space, it tracks both Steam and non-Steam games. The leaders might make you say, "LoL, WoW!"
I am fully comfortable with myself after that pun.
As you can probably guess, League of Legends is the most popular PC title, with 14.5% market share (with respect to time). WoW and Diablo III are almost a tie for second-and-third at 8.56% and 8.53%, respectively. DOTA II is next at 5.81% and The Elder Scrolls Online is fifth, with 3.78% of all game time.
Surprisingly, the tail is pretty long after that. In fact, the entire Top 20 takes up just 63% of play time, with the 21st place and lower, by definition, having less than a 0.73% share. This is a slow decline, leaving room for theoretically fifty games with Skyrim-level popularity. Several games just below the list are probably very close to one another.
I should also note that, since this is based on time, it probably skews toward long and intensive titles. This probably explains Diablo III, MMOs, and Minecraft as those games are played for hours if they are played at all. This really puts Counter-Strike: Global Offensive and, to a lesser extent, Battlefield 4 into perspective, with their series of short rounds.
Off the list since March is Titanfall, Rust, and Path of Exile. The first two are fairly surprising. Titanfall just launched and, it would seem, has not kept its players gaming habitually. Rust, on the other hand, is surprising because it is popular and, to my understanding, typically involves long play sessions.
At the very least, it puts context around Steam vs. Battle.net vs. Wargaming.net vs. Origin.