Princess Celestia just does not get enough credit. Sure, Luna is easy to love (especially when there are so many variations of her personality to choose from), and Cadance will probably end up selling more toys, but when push comes to shove I have to say Celestia is my favorite Princess. At turns majestic, playful, scheming, and always loving towards her little ponies, she's the full package.
My all-time favorite bit of plotting from FIM is probably the moment a distraught Twilight finds herself bombarded with every Friendship Report she had written in the previous season, making a show element I had taken for granted suddenly seem like a master bit of chess positioning. From that genius move onward, I've never doubted Celestia has known what she's doing... and while others enjoy attributing darker edges to her as a result of such schemes, I have no doubt that she's all sunshine and warmth, myself.