S Tier
Roger
Whitebeard (Prime)
Shiki
A Tier
Rayleigh (Prime)
Garp (Prime)
Sengoku (Prime)
Reason : Shiki was Roger's rival and had to be teamed up by Prime Garp and Sengoku in order to be defeated. After the fight, Garp couldn't even move himself, at least according to the anime version. I don't remember if there's something putting Sengoku above or below Garp though, but by the way Garp was referred to at the time ("the Hero of the Marines"), I'd put him ahead. If there's something concrete, please add it. Shiki should still be a bit below Whitebeard, since after Roger's death, the title of "the strongest" went up to WB. Correct me if I'm wrong.