9/10 Possibly the Best DLC if you have to choose one.
Easily worth it if you find it for 50% off during a steam sale!
It adds more types of planets for a little more variety to your solar sytems.
It also adds research for these planets to colonize them with Ferros being the highest level.
The most important feature is the addition of a Planet Specialization building you can choose Industrial or Cultural, which is a permanent choice.
Industrial increases Ship manufacturing speed, Trade income, and resource production.
Cultural increases culture rate, Population, and resistance to opposing cultures.
Stardock also has DLC setup so that the host shares his DLC with all player joined, an especially fantastic feature and I'm glad they included this! Same applies to other DLCs.