One of the most important dlcs for Cities Skylines.
The 2 most important are:
Industries. Thanks to this dlc you can improve your economy. The amount of money you can earn is way bigger than in the basic game.
After Dark. Bike roads are in my opinion essential for a city to exist. You get the whole turist system with 2 new zones for tourism. This adds so much to the game.

These 2 dlcs are the must haves.
3 less important but fun are:
Mass Transit. You don't really need any of this but this dlc adds so much flavor and options for your city.
Park Life. Buy it only with After Dark. It adds the whole new park building system. It boosts your tourism sector.

Snowfall. This is my personal recommendation. The climat of winter is beautiful. This dlc also adds Trams. They are my favorite choice for commercial areas.

If you have any question you can always write to me. Have fun with Cities Skylines.