If its only haki then Shanks or Primebeard.

Gotta be Goatbeard, but Shanks is a close second.

If it's Haki then shanks.

Just Haki? Probably Shanks then.

Thought it meant just normally

Haki-wise it's Shanks.

Shanks is the strongest character ALIVE. Primebeard is stronger

It's Luffy. The post says "all at their prime" meaning this is prime Luffy with mastery over his fruit and likely has insane haki just like Joyboy (likely more powerful than Joyboy's).