I have been wanting to mention this before but never had the chance so now I am making the time to do so. (note: cannot be an OC. it has to come from fan fiction stories and vids) My favorite pony outside of cannon is Blackjack The Security Mare from Sombers Fallout Equestria: Project Horizons.
the reasons I like her is that she is really funny, very random, VERY sexy, and a really good person let alone pony because no mater what, she tries to do the good thing and help ponies and yet she is no holier than though savior she is a very relate-able personality with her positive and negative moments
and those are in extremes that most people would not be able to endure.