He's a solid centre-back and vital cover considering the injury records of our other defenders.
It's pretty simple for me -
Stones - best all-rounder, should play when fit/have minutes managed to target big games
Dias - strength, leadership and organisation, best for matches that need discipline and concentration
Laporte - great passer, best for matches where we need to beat a press and find space in behind it
Ake - practically immaculate defensively, best for matches where we need to starve an opposition striker
For me, injuries permitting, we need at least one of Stones and Laporte on the pitch at all times. Our build-up play falls off a cliff without them. But they can partner with each other, or with anyone else, and those decisions can be made (after fitness issues are taken into account) based on what the match needs.
I'm not sure what Akanji's USP is. The only time I've ever thought "we could do with Akanji on here to sort this out" was against Southampton when we had Walker at centre-back.
He's a back-up, and surely one of the league's best 5th choice CBs. But there's going to be times when he plays and we really miss one of the others.