I gotta say, if there's one thing I love in One Piece the most, it's the character designs, with my favorites being Usopp, Robin and Jinbei.
Usopp's whole schtick is that he's a liar, and it shows, from his name to his nose. I even loved how he first debuted in the Syrup Village arc in his version of The Boy who Cried Wolf. I also love how he's this lanky and plain looking guy before the timeskip and changes into a slightly more muscular dude with his hair grown out.
Robin, to me, is the best woman design because back then, in the sea of Namifaces Robin was a breath of fresh air. Always did like how she liked wearing darker colors, it helped her look more unique. I'll always prefer her tanned skin to her bleached skin though. Although nowadays, Robin ain't as unique as she used to be now with Robinfaces appearing everywhere (Hancock, Bonney, Viola, Ginrummy, and those are just the ones of the top of my head).
However, Jinbei's design has to be my most favorite ever in One Piece. From his girth (like a sumo wrestler), to his oni-like face, to his lightning-shaped scar (which may or may not get an explanation later on), and his wooden sandals. I also love how Jinbei's name is also a pun on the species he's based off of and the clothes he wears. Plus, he's a Fishman Karate MASTER, you can't get any more awesome than that! Can't wait for him to join the SH Crew!
These are only just a few of the many, many outstanding designs that Oda has graced this series with and it doesn't look like he'll be stopping anytime soon.