From all of the Villians i've seen i think i have a ranking list of most powerfull creature

1. Tirek (all ponies and Princesses and Discord magic absorbed)

2.Discord ( can do anything by snapping his fingers)

3. Grogar (idk much about him but according in this episode, he is stronger than all 3 of them and Sombra(not stated but it was obviously of ep 1))

4. Chrysalis (Shining Armor's Love for Cadance absorbed defeated Celestia)

5. Pony of Shadows (Even Starswirl and the Pillars couldn't defeat is and has to be banished to stop it)

6. Sirens [the Dazzlings] (able to defeat the Mane 6 but not with Sunset)

7. Sombra (ability to give nightmares to Creatures, not strong enough against Twilight because she can reflect his attacks)

8. Starlight Glimmer (same as Twilight's Level but Sombra was able to mind control her)

9. Cozy Glow ( Weakest of them all actually, a filly pegasus)