If I had to choose between all the ponies, it would definitely be Starlight Glimmer.
First of all, I love mages. And she's one of the most powerful mages in all of Equestria. What I especially like about her magical knowledge is that she practiced her magic independently, without Celestia's school, and was able to fight Twilight on equal terms without the help of alicorn's magic or friendship, using some of her magic to levitate.
Secondly, there's her spiritual transformation. She changed dramatically over the course of the show and became one of the friendliest ponies, able to understand the Pony of Shadows and Trixie, both of whom the Princess of Friendship herself was skeptical of.
And of the main six, my absolute favorite is Twilight. I think I would spend a lot of time with these two ponies, but with Starlight I would want to study the practical application of magic, while with Twilight I would discuss the theoretical aspects (and in general I would be interested in what is interesting to read with her, not only about magic)