Followings should be your first shot of DLC playing, which do great work for economy and traffic system. They are worth best sellers on Steam.

Industries brings 4 specific regions (not governmental districts) with amazing profits. Sure, and annoying traffic problems.
https://steamproxy.net/steamstore/app/715194/Cities_Skylines__Industries?curator_clanid=44768575&curator_listid=115386&snr=1_1056_1055_creator

Another way to earn money is to reduce cost. Green Cities’ RCO styles do this well by their beautiful models.
https://steamproxy.net/steamstore/app/614580/Cities_Skylines__Green_Cities?curator_clanid=44768575&curator_listid=115386&snr=1_1056_1055_creator

While a good way to solve traffic problems is to replace 500 cars by 1 single monorail train! Although the vanilla game has provided enough methods for all 3 level public transport (according to my theory here). But we play for fun and laugh not just enough, right?
https://steamproxy.net/steamstore/app/547502/Cities_Skylines__Mass_Transit?curator_clanid=44768575&curator_listid=115386&snr=1_1056_1055_creator

Promenades provides a brand new traffic form with clean modern wall-to-wall buildings. And finally the transit metro stations!
https://steamproxy.net/steamstore/app/2008400/Cities_Skylines__Plazas__Promenades/?snr=1_16_16_creator_1057&curator_clanid=6625556

Followings are recommended to purchase once you need something new to improve your city in entertainment, education, public service… almost any aspects.

Parklife brings 4 park regions with different theme, and it’s easy for a 25-tiles-city to construct them all, in which way greatly enriches landscape of city.
https://steamproxy.net/steamstore/app/715191/Cities_Skylines__Parklife/?snr=1_16_16_creator_1057&curator_clanid=6625556

It’s hard for a 25-tiles-city to own 3 types of campuses. But don’t worry since 1 single campus could earn massive profits without any risk of traffic jams.
https://steamproxy.net/steamstore/app/944071/Cities_Skylines__Campus/?snr=1_16_16_creator_1057&curator_clanid=6625556

Tram, and it’s sorry to see so many assets trapped in few blinding winter maps.
https://steamproxy.net/steamstore/app/420610/Cities_Skylines__Snowfall/?snr=1_16_16_creator_1057&curator_clanid=6625556

Fishery. Trolley Bus. Helicopter. Better waste and water system. Sunset Harbor is a merge of city elements. It’s lucky that they’re all high-qualified.
https://steamproxy.net/steamstore/app/1146930/Cities_Skylines__Sunset_Harbor/?snr=1_16_16_creator_1057&curator_clanid=6625556

After Dark mainly adds 2 styles for commercial zone. But they’re not so excellent as styles in Green Cities or Promenades. Sorry that I forgot hubs (large port and airport) and roads (with bike and bus lane) for which it was in T3 at the beginning. Appreciate to comments.
https://steamproxy.net/steamstore/app/369150/Cities_Skylines__After_Dark/?snr=1_16_16_creator_1059&curator_clanid=6625556

Followings are recommended to purchase when you are looking for something new but not so useful.

A shortcoming of the game is lack of collaboration among DLCs. Now we have 1 passenger airport from basic game, 2 from other expansions, 3 styles from Airports DLC. While a 25-tiles-city is just capable from 1 airport region.
https://steamproxy.net/steamstore/app/1726380/Cities_Skylines__Airports/?snr=1_16_16_creator_1057&curator_clanid=6625556

Natural Disaster is innovative to introduce brand new mechanics into the game. However, it’s generally weird to introduce destruction into a construction game. You would be fun to destruct at the very beginning, and just turn it off later.
https://steamproxy.net/steamstore/app/515191/Cities_Skylines__Natural_Disasters/?snr=1_16_16_creator_1057&curator_clanid=6625556

The final 2 expansions should be one call Finance & Retreats. Both the financial region and hotels are good money-earner. They’re here for lack of difficulties.
https://steamproxy.net/steamstore/app/2148901/Cities_Skylines__Financial_Districts/?snr=1_16_16_creator_1057&curator_clanid=6625556
https://steamproxy.net/steamstore/app/2342310/Cities_Skylines__Hotels__Retreats/?snr=1_16_16_creator_1057&curator_clanid=6625556

Followings should be put on a lower priority. Buy them later.

Contain too less and not worth their price in. Please consider buying later and later.

Some CCPs, such as Train Stations, Sport Venus, Vehicles of the World are even more excellent than Concerts indeed. However, this guide focuses more on gameplay mechanics, in an other word, Expansions. Thus there’s no further detail ranking for every single CCP.

Most players turn off background music and listen their own playlist in Apple Music or Netease Cloud Music, I guess.

Followings are the must-haves.

The best things are always free. Check out here.

Sure!
https://steamproxy.net/steamstore/app/255710/Cities_Skylines/