celesteela is an amazing tank. Its typing allows it to resist a lot and avoid a few things. It can be given full defense on both stats and still provide some decent damage in physical or special. Giving it leftovers allows it to have some recover due to its recovery to live more. Or you can run assault vest and because it has access to leech seed, giga drain, ingrain. It weakness are the same as skarmory, but some are not really seen. It even has fire moves for some weird reason.
Celesteela is definitely good for ou. Shes not damage broken or defense broken, so she'll definitelybe seen a lot more in teams