Hmm, "stronger" is a bad word, I would say "powerful" (don't hit me please ). Anyway, as for the purpose of the topic, I would say this: I consider myself that Celestia and Luna, in terms of raw power, are more or less equal, because they were shown to fight together ans such, and also because it's pretty logic However, in terms of knowledge, Celestia, without a doubt, know more about magic in the technical part, like spells and stuff, so she is more difficult to beat After all, she had plenty of time to study while Luna was on the moon, and like someone said earlier, it was montioned she was pretty good against Starswirl. Nightmare Moon, of course, IS more powerful than Celestia because: anger, so not limiting herself in terms of power, plus, she was possessed, so someone would think there's a gain of power too (Dragon Ball style). So, there it goes: equals in term of power, but not in knowledge, which would put Celestia above Luna. But let's not forget that character also plays, like I said while talking about Nightmare Moon: Luna might be less kind than Celestia