If the question is: "who would most likely get a loyal harem in-universe" I'd think the answer is rather decisively Cavendish. Not because he deserves it, but because girls literally faint from excitement wherever he goes. I think various other characters might have various means to match the sheer number that Cavendish can achieve, but since all of those could very easily get as many as they want, I'm going to pick Cavendish since his harem will need the least interference to want to stay.
If you're talking about "who is closest in character to the most common and archetypical version of a japanese fiction harem protagonist", I don't really think anybody in this story is boring enough to be a close fit. Most of those stories feature an intentionally bland unremarkable yet non-ugly character, you realize. Therefore, maybe Iceberg, by process of elimination? As a competent person who started from relatively humble origins, is shallowly kind, and is otherwise broadly uninterested in the opposite sex and really boring, Iceberg seems like a good fit.
If you're asking "Who looks most like the protagonist of a lewd japanese doujin, featuring a harem" like I assume most other people on this thread are responding to: Rayan00's signature is giving me a pretty good reminder of how similarly ugly, fat, nondescript and sinister Vascoshot looks.
Though in all fairness, the majority of harem porn coming out of the japanese artist industry is not actually depicting fat ulgy bastards. It's just that the fat ugly bastard porn existing at all is weird enough to be notable. So maybe the answer of who ends up looking most like a typical japanese doujin actually ends up being Luffy, due to how nondescript he looks, discounting his scars and straw hat.