In a fight, Celestia would probably defeat Luna. She's older, wiser, and far more experienced. In a popularity contest, Luna wins any day.
Wisdom and experience will always beat youth and enthusiasm. Celestia has a full thousand more years of experience than Luna, and it's hinted on the show (and blatantly stated in the comics*) that she's simply more powerful than her little sister. *The canonicty of IDW comics is debatable.
Depending on whether you consider Luna's transformation to Nightmare Moon to be caused by an outside force corrupting her (as told in the comics) or simply fueled by a jealousy-induced rage, old Luna may have actually been pretty powerful. After all, Celestia could only defeat her with the help of the Elements of Harmony (although one could argue that Celestia was just holding back and used the Elements as a non-lethal option to stop Luna while NMM was actively trying to kill Celestia).
We may never know how powerful Luna and Celestia were before the Nightmare Moon incident. Nightmare Moon is corrupted, and Celestia never fought back against her. However, Luna's banishment left Celestia alone to fight threats against Equestria for the next thousand years. While Luna was imprisoned, Celestia was gaining wisdom, experience, and likely power. Therefore, in the show's current timeline, it is very likely that Celestia is stronger and more skilled than Luna.
If there were to be a winner it'd be Luna, solely because I don't see Celestia having the emotional ability to physically fight her sister again, seeing as it nearly broke her the first time. That said I'm a much bigger Celestia fan than a Luna fan x) Luna's bloated fanbase is astronomical while poor Celestia gets troll memes. Predictable seeing as one is depicted as having no flaws while the other is flaws incarnate and is all about redemption and character relatability, but it's still both funny and obnoxious at the worst of times x) Not to mention ironic.
Celestia wins in a pure power match-up, but I think Luna probably has some more subtle abilities (her dream-walking ability, where she enters other ponies dreams, for instance) that could give her the edge in certain specific contexts. On the whole, Celestia wins. If, however, Luna were ever able to access and control her rage and jealousy, she might gain control over her Nightmare Moon powers (which is purely fan speculation based loosely on the season five spoiler and the fact that, while Luna did lose her evil powers completely, it seems possible that she could simply undergo the same transformation twice). Sent from my iPod touch using Tapatalk
Luna for me. I think her design is way better and I like her personality more. Celestia is just kinda "meh" to me and I don't like the long because they look awkward.
How dare you ask these questions? They are sisters who had bad experience. Nekhorosho, fellow. If ya wanna know mah opinion, Luna will win at nighttime, and Celestia - at daytime. Obvious. Luna has power of Hecata and her Dream Kingdom, but Celestia is the Lightbringer. They are power-equal.
