Aura piece is correct. You can tell who's going to be the top dogs based off their intro.
Jack is an example of Oda overhyping a character up
Let me remind you what Jack did
- Fought arguably the strongest scabbards, non stop for 5 days, while they alternate getting a 12 hours break each, to a standstill
- Nuked the place and left to free Doflamingo
- Doflamingo is guarded by a Fleet Admiral, an Admiral, Tsuru and her squad, and 4 warships on the fucking sea
- He makes it out alive somehow and goes back to Zou
- Proceeds to get hit full force by the biggest known being in One Piece
- Is still conscious after this and, when we see him in Wano two weeks later, he is perfectly fine
- Gets mauled by both the strongest scabbards, in their strongest forms, and gets up later to fight in the war any way
- Would have beaten Inurashi despite all the damage he took, if not for ANOTHER sulong transformation mid-fight