Clash of Clans for me! There are over 15 of us from my hometown of Winnipeg that play together. We war a lot together and have a lot of fun. My friend, The Shoe Guy, bought 10 of us shirts with our gamer tags on them, a nice Stuugie shirt for me.
If you need to find out games that are super popular (even if only for a few weeks) just look at the most viewed games on Twitch.tv . League of Legends is almost always the number 1 most viewed, and Counter Strike: Global Offensive or Hearthstone: Heroes of Warcraft are most likely numbers 2 and 3 (Dota 2 comes up every once in a while when a big tournament is going on), but there will be some days when games like Pokemon Blue are in the top most played because of some random events like "Twitch Plays Pokemon", or other games that get good notoriety from popular streamers, like ARK or any other games that are popular for a couple days.