Sanji
A while back, I used to be a Sanji fan. He was my favorite character. I can't really single it down to one moment, it's more of an accumulation of many but I'll show a couple.
1. Mr. Prince
This is easily one of Sanji's top 3 best moments in the story. It shows off the wit and competence behind his character as opposed to the brawn. Sanji's better moments in this series revolve around his displaying his intellect. That intellect is one of the attributes that make him such a valuable component in the Straw Hats. He has been one of the most clutch characters in the entire series. What disappoints me a lot is that we have been missing this witty Sanji in the New World but instead get a more perverted Sanji with gags that are old and just out of place. Overall, this scene from Sanji was very badass and cool imo. It's a different level cool from that shows off his swagger in critical situations.
2. His introduction
I wouldn't say it was love at first sight when I first saw him but his intro definitely made set him apart from the rest at least for me. I don't know how it was in the manga, but in the anime at that time, he swooped in, sweet-talked the girl, and made an absolute embarrassment out of that Lieutenant (forgot his name but it ain't relevant). I believe that one scene fully expresses the kind of character pre-time skip Sanji was. He was a guy who was flirtatious with women, but still respected them, a fantastic cook, and someone who doesn't tolerate scumbags.
3. Feeding Gin
Last but not least, this moment. Can't truly get any better than that for Sanji's character man. The speech isn't much, but it's strong. He's willing to put aside the negatives about Gin and still feed him anyway even though the rest of the cooks threw him aside. This desire to feed any starving customer no matter their background due to how he can relate to them from his past is one of the strongest attributes of Sanji's character. We don't see it much nowadays but it's a part of him that hasn't regressed in the New World and Oda at the very least still keeps it going as shown when he baked the cake for Big Mom. It was after this moment that I knew that Sanji would be my favorite character going forward.