Rebellion is the main one you need. It contains all the content of the previous Entrenchment and Diplomacy expansions (AKA Trinity).
There is some additional DLC for Rebellion, but they are relatively minor and you can play multiplayer with other Rebellion players regardless of DLCs.
You can get the ultimate addition on Steam, it includes the first 2 DLCs (Forbidden Worlds and Stellar Phenomenon), but not the very latest one Minor Factions, which came out in December.
https://store.steampowered.com/sub/39352/
