Sunset Shimmer is a unicorn who crashes though a portal into the real world. There she is sort of like a highschool bully untill she gets Twilights crown and turns into the less pretty demon (which I think would scare childern.) After mane 6 deffeats her she becomes "friends" with them. Not how I would end it, but eh.
Discord appears in The Return Of Haromy Part 1 and 2. He brings mayhem down on Ponyville and separtes mane six and make them mad at each other. Twilight them solves the problem brings mane six back together. They then go and defeat Discord and turn him to stone. Then later in the Series Fluttershy turns Discord good.
King Sombra ruled the Crystal Empire. He enslaved the Crystal Ponies and made their coats not shiny. He returned to try to enslave them again but was stopped by mane six.
Nightmare Moon was the first villian of the series. She was first princess Luna and then transformed into Nightmare Moon. She refused to lower the moon so Princess Celestia trapped her in the moon for 1,000 years. Mane six stopped her after the 1,000 years and turned her good.
Queen Chrysalis is queen of the changlings. She appeared in Canterlot Wedding Part 1 and 2. She was pretending to be Princess Cadence how was trapped in the mines. Twilight Sparkle was first to discover the imposter and became trapped in the mines herself. There her and Princess Cadence escape and with the help of mane six got Cadence to her wedding with Shining Armor. When Cadences and Shining Aromors horns touched their love stopped Queen Chrysalis.
Trixie first appears in the episode Boast Busters. She calls herself "The great and powerful Trixie!" She pretends to be a very powerful unicorn, saying she banished an Ursamajor. She picks on Twilight Sparkles friends and asks if any is better than her. Twilight was modest and didn't challenge her. Snips and Snails and impressed by Trixie and find an Ursaminor for her to battle. Trixie fails and Twilight saves the town. In the next episode with Trixie, Trixie buys the Alicorn Amulat. It gives the user
amazing power, but coruputs them. Trixie herself can only take it off. She challenges Twilight to a duel and Twilight loses. Twilight is banished and she seeks help from Zecora. Trixie meanwhile enslaved the whole town. She had total control of th town. No one could get in or out because of a magical force field Trixie made. Twilight then challenges Trixie again with an "amult" which is Zecoras door stop. Using the power of six Twilight defeats Trixie. Trixie takes the Alicorn amulat off thinking Twilights is real, and steals Twilights. Trixie emberassed help Twilight in her performance for Princess Celestia. Trixie then runs off.
Ahuizotl is the fiction charecter from the Daring Do. He appeared when Rainbow Dash was reading the book in Read it and Weep. He tries to stop, with his cat friends, Daring Do as she goes on her quest.
Diamond Dogs are dogs who love gems. They kidnapped Rarity for her gem finding power. They took her
underground into a mine. Where she looked for gems. Rairty outsmarted them and escaped, where her friends helped her carry home he gems
Gilda is a griffion from Griffion brush off. She went to Rainbow Dashes flight school and came to Dashes house for a vist. Gida makes fun of Pinkie Pie and plays mean jokes on the town of Ponyville.
Silver Spoon and Diamond Tiara are fillys in the CMC class. They constantly pick on the CMC calling them "blank blanks." They first appeared in the episode Call of The Cutie. They appear serval other times in the series, especially in One Bad Apple. In this episode they befriend Bab Seeds, Appleblooms cousin, and take over the CMC's clubhouse.
Changlings are creatures that they change into anypony they want. Queen Chrysalis rules the changlings. Changlings, like Queen Chrysalis, feed off of love. Shining Armor tried to keep them out of Canterlot with a force field, but failed. They appeared in Canterlot wedding part 2 and were defeated the same way Queen Chrysalis was defeated.
Snips and Snails can be considered anatagonists. In Boast Buster they supported Trixie and found her the Ursaminor. They help Trixie again in Magical Duel. Snips is the small blue filly and Snails is the taller orange filly.
Flim and Flam try to put Sweet Apple Farm out of Buisness. They bring a huge machine that makes applecider faster than the Apple family can. They both get into a competition over who can make more apple cidar faster. Film and Flan win, but since the Apple families is better qulity the Apple family wins.
So who is the best villian. I don't know, whoever you like. Thanks for reading.