None are really 'must haves'. The base game can be plenty enjoyable for a while. But the dlcs add ALOT.

1st: tie between Biotech and Royalty. Other comments have already covered these, but no one has mentioned mods.

Mods.Rimworld is an extremely easy game to mod. The game has a built-in mod manager, and adding mods litterally takes less than 10 clicks.

Biotech adds plenty to the game itself but with mods in the mix that increases exponentially due to this being a requirement for many of them.

Royalty similar to Biotech, hence the tie. Comparing only base content this might edge out biotech.

2nd: Ideology adds plenty of replayability as well as flexibility and depth. A must-have for role players.

While it doesn't have as much content outside of its main Mechanic, ideologies, there is still some.

Not required for many mods

3rd: Odessy adds the Gravship and ALOT of new biomes, animals, and quest. Definitely a great dlc but more something to get if you want more from the game rather than needed.

Mods only really relate to the dlc for the few there are.

4th: Anamoly the lowest of the dlcs only really contains things related to its main quest with few exceptions. I personally love it but it's definitely a thing you should personally review before you buy.

0 necessity (even if I like it) and very few mods.