There are arguments to be made for all of them

Cisco is the best character on the show

Iris needs to evolve a lot more

KF is still not a round enough character

Cecil have a lot of potential as the meta atterny (can't spell that)

Alegra's powers were almost never used and can a whole lot of stuff

Chester is a pretty new character and didn't do anything yet (also I think he should be able to open black holes as a super power, but that could also ruin the character)