Rarity of course! Who else could it be? She's stylish, proper, generous, and even though she's lady-like she can handle herself in a sticky situation. Also, I like her expressions like when she gets excited, angry, or whiney.
My second favorite (first before I changed to Rarity) is Fluttershy. She's just so sweet and kind. What's not to love about her? (besides the fact that she's weak and scared of her own shadow)