I guess to answer this properly like the other girls. I've always been drawn in by tragedy, depth, and emotional moments in anime. I may be slightly bias as I finally got into the show/manga from WCI to begin with.
But Sanji is the single most well written character of the story thus far. Even in earlier content, when looking away from the gags, you had the Mr.Prince moments with stealth, coolness, last minute heroics, and when digging deeper hints of insecurity and low-self-worth. After WCI when you got the full story of his past, it made those earlier moments even better as you can sit there digging further into his character, going back and looking at small things that were foreshadowing, buildup, or explanations for how he acted in a scene.
Such as Jaya shadowing, his first responses to Ace as a brother to Luffy and the way the panels were placed to reflect things on certain words. The way he was introduced, the deeper feelings that you can sit there and analyze for hours with Zeff (Zeff while being a savior and parent, was still another abusive situation and while better was not actually the best role-model, I don't think any parent in the show is).
Aside from that, WCI sealed the deal. He took a character with hints of depth that if you weren't looking for it you would have missed, as saw for nothing but the gags and fronted happy go lucky when not insecure personality and completely broke him down to the bare basics in a situation with no outs while revealing the situation he came from, had to face, and how that affected him throughout the arc. Having moments that I still re-watch/re-read of his fight with Luffy being done as a way to sever and make him not deserve being in the crew/lose the respect they had for him which backfired, as the moment where he learns it wasn't his fault for his family status when Reiju finally reveals things, the moment where Luffy says 'that's just the way you are' and says what his family never said to him showing acceptance.
You can still literally go back to all these moments and find things you may have missed, or interepret more deeply now as the story moves forward, and Wano brought even more depth with it between his self-worth finally starting to repair itself and I look forward to the rest of that development.
In terms of characters in a shounen being able to come to the real world, Sanji is the one that feels the most real of the cast.
I always liked Zidane from FF9 for that moment where Garland completely broke him down in-game. The happy go lucky character completely destroyed. But looking back, Sanji's moment of that was 10x worse.