I have no love whatsoever for images that features this many characters, but still fail to include the seventh part of their group. It implies that Spike's tight bond with Twilight, plus all the support and devotion he demonstrates, mean nothing.
as brilliantly detailed as it looks, the only part that looks out-of character seems to be Sombre, Chrysalis. I don't really see him as a lazy yet smirking character. And Chrisylis looks more like a spunky high school girl than a villainess.
Very good quality, I can tell you put a lot of effort into it and it shows in many details. I think you've nailed RB's essence with just her smile alone and Pinkie Pie's mustache is a great touch. I love the look on Discord's face as well... though;
Pinkies right-front hoof is oddly misshapen... unless that's her back leg... It's a poor place for RB's tail, Pinkie pie just blends too well with Fluttershy's mane to have more of her hidden from view by RB, it makes her blend together too well into the background, also; Pinkie's hair looks segmented rather than curly or poofy.
I hope you don't take offense to anything I've said; you are definitely a highly skilled artist. Even the best can improve but I'm just one guy offering my individual true first impression. I'm not attacking your art at all, just giving a critique... I always thought it's better to hear a full opinion on art rather than just "OH I LOVE IT, IT'S PERFECT NO FLAWS"..... ALL art has flaws.