Cities: Skylines is pretty much the best city builder ever made, for two reasons: it's very fun to play, and reading Chris Livingston write about it is among my greatest pleasures. If you haven't picked it up yet, you can do so now for $10 as part of the new Humble Strategy Simulator Bundle.
Although, if you have picked it up, the bundle (opens in new tab) will still be of interest. The "pay what you want" tier includes Rebuild 3: Gangs of Deadsville, SimplePlanes and Out of the Park Baseball 18. Pay above the average (which is currently $6.15) and you'll also get Mad Games Tycoon, Plague Inc and TS 2017 Train Simulator. But pay the $10 and you'll get all those in addition to Cities: Skylines.