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 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.