Celestians
Celestians are the best of the best within an order of the Adeptus Sororitas. To earn the rank of Celestian a sister must have proved herself on dozens of battlefields and must be skilled with every weapon in the Ecclesiarchy's armory. These ladies are some of the hardest SOB's (DOB's?) you'll meet save for the space marines. Off the battlefield they have many of the duties of a sister superior in addition to serving as go-betweens for the Cannoness and the other sisters of her order. They also serve as bodyguards and advisers to important members of the Ecclesiarchy.
Despite their apparent badassery in the fluff, on the tabletop they're a bit indecisive as to where they want to be. They're geared to be a heavy-support/elite squad, basically working like regular sisters but 'arder and killier, however they're an assault squad with no melee weapons, yet their Act of Faith is close-quarters oriented. If you really want to play them fluffy you can give them a heavy flamer, flamer, combi flamer, and a priest. March them up the board burning heretics and taking names. For the occasional grizzled neckbeard who still plays sisters celestians aren't a common choice, as they're competing for the sister's vital elite slots while not being terribly efficient at what they do, considering their point cost and whack AoF.