The first thing everyone should realize first, is that we basically talk about headcanons, because the show gives us really vague information about ponies' actual power. That being said, I would choose 5 ponies to compete in that matter: Celestia, Luna, Starlight, Twilight and Sunset.
If we believe (and I believe) that Royal Sisters are some kind of demigoddesses, that puts them in this squad. And even being Luna fan, I have to notice that Celestia is slightly more powerful in terms of common magic, and not dream thingie. I like to put it like this:
Celestia = 100%
Luna = 90%
Nightmare Moon = 110%
Twilight got her cutie mark for magic, represents an Element of Magic, and spends most of her life studying magic. Additionally, she became an alicorn greatly enhancing her skills. Everyone knows that the book horse is magic horse, so that's why I put her here.
Compared to her we have Starlight, who brings a lot of chaos into this organized mix. We could say she's more powerful than Twilight because book horse could not stop her, but because we're talking about personal interpretations mostly (as I mentioned), I will say she's not more powerful. When she used Starswirl's spell to time travel, she merely worked on a ready base which was given by Starswirl and then by Twilight. And, Twilight couldn't stop her because it's harder than most people think to stop someone if he has already started and is determined not to stop. Twilight probably could stop Starlight only using damaging magic to defeat her, and that's not what we want to show. But, I still value Starlight as really slightly less powerful or even Twilight's equal.
Why is Sunset here oh my god, she's horrible and blah blah. Actually, we do know that Sunset was Celestia's personal student just as Twilight, and that means a lot. We do know she has huuge magic potential and was also well trained. She didn't do things like Starlight, but this argument is mostly invalid because these deeds are not defining somepony's magic power, but what opportunities he had, and simply what was his individual story. I do think that Sunset probably has more power than Twilight, but she never actually got to know the power of friendship and harmony, basically drowning in her arrogance. That does not make her raw magic potential any lesser though.
So, my (highly personal) view on this matter is:
I don't really know, but I do know that all 5 ponies I mentioned are on the god tier list. If I had to, I'd go with:
1.Celestia (100%)
2. Actually Sunset Shimmer (95%)
3. Twilight and Luna with a draw (90%)
4 Starlight (85% or 90%, not sure)
Trixie's just a simple trickster, not a true mage you fanboy.