TotallyNotNyx

In terms of Daybreaker being stronger than Nightmare Moon (in the present day) : probably, but not by that much. 

Some of it may be because of Luna spending a 1000 years in the moon, so she's probably inferior to her sister in knowledge, skills and magical power, which would be reflected in Daybreaker being slightly stronger than Nightmare Moon. Plus Daybreaker harnesses the power of the sun itself (seen by the fiery plasma thingie that swirls around her, which she then shoots as a beam), while Nightmare Moon couldn't seem to do the same thing with the moon. Nightmare Moon's moon powers did seem to have more defensive potential than offensive potential. 

Daybreaker is literally the epitome of confidently evil villain. Her whole demeanour portrays that she did not believe that Nightmare Moon or Celestia were a threat to her. She was literally laughing the whole time during their battle, to the point that I'm starting to feel sorry for Nightmare Moon for having such an evil sister. 

No matter how bad Nightmare Moon may seem to be, Daybreaker seems to be worse. 

Nothing

@Spacetime Vortex @ShadOBabe I'm not a fan of how Past Sins portrayed Celestia. The story made me so angry at her after chapter 10, and I didn't want Twilight to forgive her at first. But I also really loved that because of that we got to see just how far Twilight would go to protect Nyx, even tearing chunks of stone out of the wall as weapons to hurl at Celestia.