@shadowfalcon76  
Of course, that in itself was kind of suspect, given the established world. The Royal Sisters were living gods who could sling the sun and moon around with a thought and a raised eyebrow, tasks that prior to their ascension required hundreds of the most powerful unicorns in the land to work in concert and pushed them to their absolute limits.
 
Then comes Purplesmart, freak mutant powerhouse prodigy who canonically has the most raw power of any unicorn born in centuries. She goes to a special school for gifted unicorns and is known all over Canterlot. It is established in canon that she is the Pro from Dover, the most OP ever. She spends most of the series rescuing Celestia from threats that a living god ought to have been able to reduce to a scorch mark on the castle’s marble floors with a dirty look before they got three words into their villain monologues.
 
Then comes Glimglam, whom no one had ever heard of before, who hasn’t had the special training in the school for gifted unicorns, who spends several episodes walking all over all of them and everybody else put together with hobnailed boots. She’s not just a unicorn. She’s not just a god. She’s a Timelord with a TARDIS concealed in her horn, and in a fit of pique she uses time travel paradoxes to destroy the world. Several times. Over and over. Glimglam is basically The Master from the Doctor Who series, if The Master were a cute cartoon pony and not a mustache-twirling evil Timelord. She’s callous, manipulative, diabolical, much smarter than a certain purple pony established to be Wile E. Coyote level soooooooooopergenius, and completely psychotic.
 
Granted, you have to expect some deus ex machina bullshit in a setting where time travel is canon, but she is the plot-armored poster child for Diabolus ex Machina. She is a manipulative cult leader, a psychopath willing to destroy the world–and Purplederp, who is Asperger’s in pony form, is the one who is able to talk her out of it. Not Pinkie, who is actually good at making friends. Not Rarity, who has actual social skills. It’s Purpledrank, who lacks all instinct for social interaction, who saves the day, by talking the psychopath who’s willing to murder millions over a grudge into giving up evil and being nice. Doubleyew tee eff, writers?
 
This is why nothing after season 2 is canon. Change my mind.

@shadowfalcon76  
Agreed, Starlight was one of the coolest villains. As a good guy she was fun, but not quite as badass. She did have some cool scenes, but that’s about it

NGL, if it weren’t for the literal walking forces of nature that required absurdly powerful artifacts, divine intervention, and deus ex machina to deal with that go by the names Discord, Tirek, and Chrysalis, it most definitely would be Starlight Glimmer that would be the most powerful entity in all of G4. Glim Glam, as just a unicorn, is OP as fuck, and she’s pulled off some of the most redonkulous shit in the series ever. Being able to forcibly rip someone’s destiny physically from their bodies, tear the space-time continuum a new one in 50 different ways, and wipe Discord off the face of the planet with a DBZ style laser (and not taking 15 episodes to do it), really attests to just how much power is packed into that pony.
 
She was genuinely the most threatening villain of the series before her redemption. She was still hella threatening after, too, but Jesus Christ the shit she got up to when it was time to throw down when she was one of the bad guys. All of existence owes Twilight a helluva massive “thank you” for even being able to talk her down, cuz she was sure as shit not overpowering her.