The 2 options that I am thinking about are the Flying Nun or the Immolator Canoness .
The Flying Nun: She can be pretty awesome but you gotta run some Seraphim with her to give her some support. For 151 points you can give her a Jump Pack, Blessed Weapon, Inferno Pistol, Frag grenades, the Cloak (2+armor),
Mantle(insta death saver) and a Book(morale saver). She can be pretty effective at blowing up tanks and hitting weaker units. I think she needs a Seraphim squad to help screen her advance before she goes off hunting but I guess you could run her solo if needed. She is one of my favorite units and I like to use her a lot. I gotta post a pic of my custom mini that I put together and its one of my favorites
Immolator Cannoness: I have been running this recently and really like it. I threw an Canoness into an Immolator with a Celestine Retinue. Outfit her with an Eviscerator, Book and Plasma Pistol for 90 points and give the Retinue a Hvy Flamer, Melta and a vet superior with a Eviscerator. I also put the TL MultiMelta on the Immolator just to add in a little extra Firepower. Its a nice little unit and can be pretty tough. Since the size is small you can make all your saves invulnerable pretty easily and with 2 Eviserators dish out some damage. I especially like having the 3+ to hit with the Celestines on the CC.
I would love to get some input if anyone catches a glimpse of this post and hear what works for other players. There are not too many Sisters players in my area so hearing any new ideas or opinions really helps. I am starting to think that the Girls are pretty competitive in 5th edition and are a stronger mechanized army than people think.