There are lots of great actions these days. No one can tell you with certainty what action will work best for you without providing more information about what exactly you want to accomplish:
- Does weight matter?
- What is your preference for surface treatment/coating?
- Do you want CRF, or is push feed fine?
- Do you want a "quick" barrel change feature?
- Do you desire shouldered prefit barrels?
Etc, etc, etc.
Here is my personal opinion:
- No experience with KS Arms, nor do I personally know anyone that's had one. So no comments on that action.
- The experience with Curtis in my shooting circle has been less than stellar, personally not an action I would recommend.
Defiance:
Defiance Deviants are great actions. My .300NM is built on a Defiance Deviant, and my first precision rifle was a Defiance Rebel. In the past, Defiance's have had pretty tight clearances, which made them not the most reliable for adverse conditions (my Defiance Rebel really struggled in really dusty conditions). I haven't had issues with my new Defiance Deviant, but it also doesn't get exposure to adverse conditions like my previous Defiance did. I can't speak to the new Defiance's in regards to their clearances and ability to handle debris. I imagine they are better. I know you can spec them in a way that makes them more tolerant of such conditions, and they may just come that way from the factory now. If you seek custom options, Defiance is a no-brainer.
I would lean towards the Defiance, but if you are spending considerable amount of time in adverse conditions, I would talk with your gunsmith and/or Defiance to ensure the action is setup to tolerate such conditions.