I think we've had one of these blogs before but that was about your least favourite pony in general. This is your least favourite mane six pony. Note: I said pony. Therefore, Spike is not an option. Until he gets transformed into one someday. Feel free to post reasons too.
Mine is Twilight. I don't know. After becoming an alicorn, she's been shoehorned into almost every episode for no apparent reason even when she's not supposed to be the focus. I know she was the focus for a while, but now that the series is focusing on other ponies, she doesn't have to be in every episode. That's my biggest complaint with Twilight. I like her aside from that.