Discord's not better than HIM or Mojo Jojo or the Gang Green Gang, or Fuzzy Lumpkins.
He's better than all the PPG villains combined by leaps and bounds. He isn't even a villain, and that is why he is better. Chaos doesn't really have an alignment or agenda, he just happens to be against the protagonists when he is introduced. Also, he when he is on screen he is a scream which is much more than I can say for PPG villains.
Nightmare Moon is my favorite because of the character design and tragic back story.
"The love sucking shapeshifter" appeared in the wedding episode that everyone thought was going to be abysmal. She is popular because its close to Disney villain territory. She even has her own Disney villain song that has
15,980,945 views on youtube. Pretty impressive if you ask me.
Mojo Jojo: redundant monkey with a stupid billowing cape
HIM: don't even really know what he is besides creepy and annoying to listen to
Gangreen Gang: edgy teenagers?
Fuzzy Lumpkins: had to look this one up since I'd forgotten. The hillbilly sasquatch thing? I don't even...
And the other MLP villains are who? Diamond Tiara and Silver Spoon?
I'd call them more foils than atagonists or villains, just kids being bullies.