For me, the most recent large installment of C:S DLC is mostly optional. It will not raise your experience to another level, but it will give you a few nice new toys to play with in your city-sandbox and maximize options.

Sunset Harbor is a really interesting expansion to Cities Skylines, it's add a lot of features that has been previously been there through mods (industrial fishing, metro above ground, etc.) but it does feel like the pack is a bit all over. Despite that, I do like this pack and it's a good addition to the rest of the game.

This is a minor dlc, with some average changes, but it is recommended for only hardcore cities skyline players.