Mane6: Pinkie Pie. Ses just too hyperactive for my opinion and talks too much. Background: Trixie, even though shes not "evil" i don't like her cause shes mean in two episodes and a show off Snips and Snails for being a bunch of bad made couple of idiots.
Still Rainbow Dash of the Mane 6, but I guess my 'Least favourite' is very scarce because I even like Diamond Tiara really. She creates conflict and sass. Don't really care much for Snips and Snails. They're dumb for the sake of being dumb. Rainbow Dash is cool, but still aint as sassy as DT. Fave is Fluttershy. I know that's obvious but I wanna throw it out there.
I can't answer such a question, they're all colorfull, joyfull ponies. There ain't anything as least favorite. With the exception of random, background charaters that are given a few lines of text. Exemples, that pony carrying luggage to Rarity's suite in Sweet and Elite really is annoying. He has no character developpment, no personnality, he's just there to fill the empty space. Or, there's Mulia from MMMystery on the friendship express, who had an annoying mule-inspired voice, and isn't referenced in any other episodes (like Gustave Legrand, but he had a personnality, and a french attitude. He was awesome.) Anyways, i don't like characters that aren't shown much, and don't have a particular personnality or developpment. I consider them to be background characters that are given importance for no reason.
I can't say that I dislike any of the Mane 6 less than some one of the supporting cast. Though Pinkie Pie and Applejack occupy bottom tier for me when regarding just the main characters. Snips and Snails. Together. It's not so much what they do, it's that Snails is like grinding my teeth on a chalk board and I feel dumber just hearing Snails talk. I was not happy when they were given henchmen duty in Equestria Girls.
The Cutie Mark Crusaders. I don't know if any of you are Tropers, but I HATE Tagalong Kids. The Junior Ghostbusters, Scrappy Doo, Zack the Fifth Turtle, Argent and Risk and Prysm and Fringe, Harley Keener, Chibi-Usa, Wendy and Marvin, Justin the Blue Turbo Ranger... ...they are not as strong, as smart or as powerful as the adult heroes, they are unneeded additions to a cast that is already perfect, they serve no purpose in the overall story, and they are only there in a half-assed attempt to appeal to a demogrphic that is already hooked on the show and doesn't need a little kid hanging around with the main characters so he/she can have a character he/she can "identify" with. Such is the case with the Cutie Mark Crusaders.