There are many elements, so there are several criteria.

Here is the ultimate overthiking process :P

The art style is the same, and I love it, it looks so royally beautiful (if this descrption makes any sense to you). So that doesn't decide anything.

Next I went for the writings, being the type that is so much about meanings and messages. I dismissed those implying fighting and falling apart, if the writing is the criterion, I want it to say something I like thinking about.

Then I chose based on the characters that are on them (which I could've done straight away because I prefer Four, but I wanted to consider other factors too). So, Four it is.

I think i like allegiant - the shades of blue are very vibrant. i also always interpreted the 3 feathers it includes as symbolism of her death, from birds to feathers, if that makes sense. i don't think that's what it was intended as lol, but i just thought of thattt.

If I'd only chosen based on the color, I'd choose Allegiant too, it's my favorite color, and I like the choice of shades on all the covers.

Oh, interesting! I can't tell what the things on Insurgent cover are, it'd help me see if I believe your theory was the intetion. It could make sense depending on what they are. But I enjoy finding meaning where there isn't one lol, so I love it when someone discovers it within an existing thing.

My thoughts weren't quite as deep, lol. I didn't like the silhouettes on Allegiant and Four. And the arrangement of the Insurgent cover just felt off to me. Divergent it is!

It was hard since they were all similar and in a very beautiful artstyle.