What can I say, but 8th ed Sisters rock. The Sisters of Battle have always been a short ranged army with its reliance on faith and unit combos to win battles. In 8th the pattern has stayed the same, but the rules have really favoured them in this edition. Flamers most of all (the removal of flamer templates is still a sore point with me, but that's a different matter)
Saint Celestine and her gemini twins, are expensive... But Dammmmn!!!, worth every single point. (I'm not kidding, she has won me matches, and has turn absolute kerb stomps of my army by my opponents into only tight loses. Only recently in a 4 way battle of HQ's only, She wiped the floor with a Coldstar battle suit Tau Commander, a Cataphractii Armored Space Marine captain and old Abby himself)
Exorcist: once again expensive, but if you want long range fire support. Its your only option. besides the possibility of throwing down 18 wounds in one volley is hard to ignore
Immolators: Now this is tricky. It depends on the load out. As stock I cant praise them enough. With the heavy bolter option, still not to bad as it gives you some range. but the multi melta package... well... I would say almost too expensive. I still field one.
Penitent Engines: Another unit that's worth its weight in points, until the enemy realises what it can do then they'll target it. So it goes from a nasty surprise to a bullet magnet. (had one get charged by a swarm of genestealers 20, 3 turns later it walked over their corpses LIKE A BOSS)
Battle Sisters: they have always been the back bone of this army, but now with the new acts of faith they have been reinforced with some steel.
Repentia: have not been able to see if they are any good, apart from being bullet magnets. they were shot to pieces in the match I fielded them in before they could really do anything.
Saraphims: I hate to say this as they are one of my favourite units, but in 8th. They seem to lack a little of the punch they use to have. The acts of faith system does not help them greatly like it did in previous versions.
Celestians, Dominions and Retributors: I cannot comment on as I haven't fielded any ... Yet
Imagifier: necessity... no joking, they are expensive. but if you want an army to work... well!
Hospitaller: luxury..the points cost versus their ability, difficult to choose. I would say yes if their bubble was bigger.
Diologus: why bother.. no really. I could see them being useful if you take large sqds. but thats it