If I had to choose which to watch again, I think I'd rewatch Annihilation, but that is tempered by the fact that I only just recently rewatched Ex Machina. I am certain there are things that I "missed" on a first watch of Annihilation - and with many more books in the Southern Reach to read through - which I will, I feel it will inform the film more.

I did read that Garland and the screenwriter(s) didn't read the sequels to Annihilation and embellished on their own terms, and I think that they did a bloody good job!