My opinion for the Top 5 villains. You can comment about your opinion for the Top 5 villains, but cite the reasons if you like to.
TOP 1: The Dazzlings
=>Tried to take over Equestria, but banished into the human world by Star Swirl the Bearded. Attempted to take over the entire human world after feeding off plenty of energy and Equestrian magic. Their plans however failed as they were defeated and their pendants broke, which they lost their ability to sing.
TOP 2: Tirek
=>Escaped from Tartarus to consume every pony magic and take over the entire Equestria. Succeeded in stealing all pony magic, but defeated by the Mane Six.
TOP 3: Queen Chrysalis
=>Tried to take over Equestria with an army of changelings while she disguised as Princess Cadance. Defeated after Princess Cadance and Shining Armor blasted her and the changelings with the power of love.
TOP 4: Nightmare Moon
=>Overcome with jealousy, Luna turned into Nightmare Moon to destroy Princess Celestia, rule Equestria and bring nighttime eternal. Turned back to Luna after she was defeated by the Mane Six.
TOP 5: King Sombra
=>A tyrant king who came back to take over the Crystal Empire once more by forcing his way past the protection barrier around the empire. He was defeated by the power of the Crystal Heart.