Rarity, Princess Luna, and Derpy! I think I love these characters the most because they tend to get misunderstood.
Rarity may come off as prissy and fussy (Applejack sure thought so!) but I remember her kicking a manticore in the face and cutting off her own tail to fix a mustache. And then, later on, offering to make her friends free dresses (how many ponies would do that?). And then, when they did not appreciate them, she worked even harder to give them exactly what they wanted and nearly ruined her career. I find her very complex and intriguing. I am happy she is the element of generosity.
Princess Luna - the episode Luna Eclipsed sealed the deal there. I loved that episode.
Derpy - the "save Derpy!" campaign is how I came to MLP in the first place. I will always love Derpy!