Cities Skylines is alright but my problem with it from when it launched is after about an hour into the game it stops being challenging and just becomes a zen city-builder-themed garden. There's really no chance of losing once you build any kind of cash flow, and as a result there's no obvious goal other than 'make big city' and then subsequently 'manage the traffic'

SimCity on the other hand is challenging to keep going throughout. You need to make thoughtful choices on what you're building next and where or else due to some quirk of the Agent system you're going to be on the brink of collapse (Why won't the power go down this one street??)

Obviously there's room for improvement on the SimCity front, but my point is that if you're looking for a challenging city builder, Skylines is not it unless you wanna spend time configuring mods.