Tier 1: Gol D Roger, Monkey D Garp, Monkey D Dragon (also would be prime Ace and future prime Luffy and at least Blackbeard) ... sue me

...mPS it's kinda weird there is no extra special Marine equivalent of Luffy like D Garp to D Roger, Coby or Smoker or any of the Admirals for that matter don't quite hold up to Monkey D Garp from what I've seen
Tier 2: Whitebeard, Shanks (maybe Sengoku) (prob future Law)
Tier 3: Sengoku, Akainu, Kizan, Kaido, BM, Mihawk, Rayleigh, Blackbeard
Tier 4: Fujitora, Kizaru, Ryokugyu
The gaps between tiers from 1-4 are imo almost negligible, between 4 and 5 there is a slightly more noticeable gap
Tier 5: Marco, Sabo, Beckman (maybe Kuma and Magellan)
and remember, this isn't Naruto and Roger isn't the SO6P, Roger can't just wipe the floor with an admiral, imo Akainu or Kuzan could push him very far and imo Roger couldn't win against Sengoku with anything less than hight-extreme dif