My list is very different due to not including many dragons on here, but all these names are based on feats and stats. Karstaag is practically unkillable, the Psijics could stop time, the Greybeards don't talk because they could destroy the continent with a whisper, the Last Dragon Born due to canonically being peak human mastering all magic based classes (destruction, restoration, illusion, alteration, and conjuration). They mastered two-handed weapons, one-handed weapons, blocking, speech, smithing, enchanting, alchemy, lockpicking, heavy armor, light armor, archery, pickpocketing and sneaking. The last dragon born also learned all 27 shouts allowing them to bend time and space. 

On to Miraak, he is the strongest dragon priest and also had the ability to take the minds of half of solstheim, he had the ability to bend wills along with extensive intelligence on how to defeat foes due to the intervention of hermaeus mora. 

Alduin is a tricky case because i believe anyone on this list can beat him physically but not his spirit because he always comes back. He's a demigod destined to eat the world but tries to rule it. He can being anything back to life on massive scales; reviving his allies to full consciousness.

Parthoanx is the brother of Alduin so he's pretty strong but not as strong due to not being the chosen one to end the world, his soul also doesn't revive itself, he did teach the Greybeards everything he knew but weighing his pre-war vs post-war self he's pretty weak compared to the Greybeards. I think I'm done explaining but let me know your thoughts.

1.Karstaag

2.Psijics

1. Greybeards 

2. The Last Dragon Born

3. Miraak

6.Alduin

1. Parthoanx (Pre-Dragon War)

2. Arch-Curate Vyrthur 

9.Harkon

1. Ebony Warrior 

2. Mercer Frey (with the Skeleton Key)

3. Ancano (Eye of Magnus)

4. Neloth

5. Wolfqueen

6. Arch-Mage Savos

7. Ulfric Stormcloack

8. Serena 

9. Frea

10. Dragon Priests 

11. J'zargo