I think Royalty is a must have for me. Combat is a big part of the game and it improves it a lot.

Psycasts give you options beyond a big firing line. Ultra tech swords, hammers, shields, and much better armor make melee useful. With those combined you can play a tall colony instead of a wide colony.

It also adds more interesting reward quests.

After that ideology vs boitech is what interests you.

Ideology allows you to change what makes your pawns happy or sad and what they are good at. It also gives access to reverse ageing and fixing scars and missing parts.

Lots of role play with mechanics. So you can be transhumanist, mad max raiders, drug cults, or nature loving hippies.

Biotech adds child birth, genetics, genetic engineering, vampires, robots and pollution. The genetics are somewhat OP but fun. The robots ate kind of meh.

Pollution is a fun idea but implemented in a very dumb way. It's basicly stockpile this stuff until late game when you get a magic pollution destroying machine. I would like a mid game option that gives something to do with it even if it's complicated and time consuming.

I'm partial to Biotech. Mostly in the sheer versatility, depth, and replay-ability it adds to the game, have you ever wanted to:

• robot army? Sure!
• raise children? You got it!
• mess with genetics? Why not!
• diversity beyond the standard issue human? Done!
• become a vampire cult? It's an option!
• weaponize pollution? Absolutely!

The other DLCs aren't bad, but aren't quite as impactful/ noticable compared to the banger that is Biotech. Quick rundown for the sake of fairness:

• Royalty: adds a Royal faction with advanced technology and access to psychic powers. Unique win con is hosting/ protecting the emperor at your colony.
• Ideology: adds religion/ beliefs, that can shape how your colonists feel about various things (being underground, what they eat, how they feel about technology, and more) and other people. Also adds trees that spawn a dryad helper when your colonists take care of the trees. Unique win con is a treasure hunt quest line that has you partially restart your colony a few times.
• Anomaly: adds a horror twist to RimWorld. Fight and study monsters, raise the undead, and uncover horrors beyond comprehension. Unique win con is a study of the void and the entities beyond it.

I haven't had a chance to play with the new DLC yet (modder's curse), but it's looking to be a serious contender for my favorite. Odyssey lets you build a ship that acts as a mobile colony and you can travel both the planet and it's orbit collecting upgrades for your ship and other unique items. Unique win con is an ultimate showdown with the mechanoids in space.

You don't have any? Do you feel like you have decent grasp of the base game? If not, I wouldn't worry about adding anything extra yet. They all add significantly more complexity and that's probably not good if you're still trying to internalize the core mechanisms.

If you're ready for an expansion, I would recommend Royalty first. It adds more types of missions, royal titles, psychic powers, and new gear (clothing, armor, weapons, etc.).