For me personally:
1. Becky
2. Sasha
3. Bayley
4. Charlotte
And that's not me bashing any of them in particular, because they're all very, very good, and the positions have definitely shifted over the last few months (I thought Sasha was the best for a good while to be honest).
Becky has been very good on the main roster with her promos, feuds, matches, nearly everything, she has ascended to a level that they clearly didn't expect her to reach, and, given that she always appeared to be the odd one out, she deserves a lot of credit for standing out like she has. She has a natural babyface appeal to her and has shown a brilliant ability to connect with fans.
Sasha in my opinion was the best performer out of everyone in NXT in 2015, her run was very memorable and she was a fantastic character. Since the call ups however she has struggled to hit that same level, which i don't exactly totally blame her for, because the booking team is dreadful with women who are outside of the title picture. Her promos and character haven't really hit the mark, though. She definitely needs to be a heel too, Sasha is just... not a face.
Bayley has done a great job of carrying the divsion down in NXT without the other three, and is an incredibly great babyface, who has bags of potential in the future. It pains me to put her third because i do love her but i think she lacks a little in the skills department compared to Sasha and Becky, but she is very, very good and she's still easily material to become a top face of the division.
Charlotte's initial main roster face run was a disaster, she never got the crowd on her side post the NXT hype, her ring work was sloppy and her promos were really painful to listen to. Since she's moved into her natural heel role though, she has done very well for herself in matches, and her promos feel a lot more natural. I still think she needs to dump Ric, but a lot of credit has to go to her for her evolution.
All four need to be key figures in the division in the future, that's for sure.