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 film I 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