I'm having a little trouble figuring out my favorite pony.
I'm starting to think it's Rarity. She just cracks me up sometimes ("But I thought you waaanted whiiiining!"), and just appeals to me more.
I love the whole of the mane six, but Rarity ended up being the most interesting.
Discord is my overall favorite character, though. Followed by Princess Celestia, probably.
I'm probably in the same boat. All the mane cast are just so appealing and likeable, it's nigh-on impossible for me to pick one as an absolute favourite.
Though for otherwise characters, it would be a tie between:
Discord- Q in chimera form, with his sinister nature and hammy acting turned up to 11. Need I say anything more?
Celestia- Say what you will about the so-called troll/molester/tyrant. I like her!
Snails- He's obviously Canadian. I'm Canadian. End of.
Derpy- Her fanon is just so sweet and funny, and the voices given to her by fans round her off perfectly.
Dr Whooves- Dr Who as a pony. Nothing more needs to be said.
Lyra- Her actions and behaviours in the series are just hilarious. Just look at the look on her face when a parasprite eats her pie!