Marco has a few "durability" feats i guess... but garp rose serious questions if his healing ability even works when he is hit with haki....
It is not far off if we assume that Garp, who unlike WB was not sick on MF war, is most likely still one of the best haki users there are in op world. Reason for that is that only him and WB were equals to GR, and unlike WB, Garp does not seem to be a fruit user, ergo his lacking destructive ability is safe to assume to have been overcome by sheer massiv haki. Moreover Garp already was discribed as the Demon of the marines iirc.
Therefore i doubt that any other character, except Shanks, whos haki was already pointed out to be special, and mayhaps Kaidou who seems like a brawler to me (pure speculation though), has anywhere near the amount of Haki Garp once was capable of in the past, not necessarily in MF war anymore though.
Furthermore Garp made it pretty clear that he at least pretty much stood hugely above Akainu by saying to Sengoku he'd better keep Garp down otherwise Akainu would get murdered, although he pretty much came to his senses shortly thereafter,meaning not to betray/weakening the marines by such a deed.
I may be biased here, but for me only Garp was able to inflict any damage to Marco (prior to kairoseki, the harm he received during that time for me has hugely affected his fighting prowess in generel, since i doubt that someone that normally does not feel any damage, because of DF, has a huge pain resistance), therefore his fighting capacity, at least for me, is still hugely underrated.
Wb said he was still the strongest.
Wait what? All i remember is him stating that he could not stay the strongest forever...shit its been a too long time, i think i should reread op.....
BTW i dont mean to state/present what i wrote as fact, just things to consider.