Wouldn't it make sense for him to be the weakest?
The durability of the serpahims should all be equal as it based off Kings DNA. The DFs they have are just for variety/what Oda thinks looks cool.
The only factor left is the DNA of the original Warlord meaning the hierarchy still stands i.e. Mihawk > Kuma > Hancock > Jinbei.
Mihawk is a regular human with great Haki and one supreme blade.
Seraphims so far haven't shown Haki, which is tied to spirit and ambition. S-Hawk also has just a replica of Mihawk's blade. Nowhere was it stated to be as strong as a supreme blade.
Kuma has a special race that we don't know how much impact his physical prowess.
Hancock comes from the Kuja people, which are known for their haki as well. She also possesses CoC.
S-Shark comes from Jimbe, a fishman, that, in normal conditions, is physically superior to regular humans in physical attributes alone.
Nowhere was it stated that the Seraphims rank accordingly to their "originals".