I would say each have their strong points.You might be the first person I know who found Arrival simplistic.
I really liked it, Arrival was great too but it felt a bit simplistic.
Even I had to look at a video on YouTube to have a full understanding of the filmI definitely need to watch Annihilation if you're putting it above Arrival.
Arrival was very atmospheric and told this interesting story wrapped around the main character.
Annihilation felt a bit more "adult" focusing instead on human emotion while at the same time having this huge alien presence.
Both great films
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