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.