Best example? Enel. Enel is someone who would be admiral level or a pirate with at least a bounty of 500 million as confirmed by oda, he has a super strong fruit, intangibility, can read movements of anyone within 50 km radius, and his attacks are so overpowered he can kill nearly anyone in a single attack. Luffy lucked out with being rubber, or else he would not of won. Not only that but even with that fact enel still found a way to beat down luffy using aftereffects of his fruit(electricity to melt gold etc), till luffy finally won.
Crocodile, luffy using water then blood to solidify his fruit.
Ussop besting number(4?) and nami constantly winning not with strengh but strat~.
Sanji is always behind the scenes looking for ways to sabotage.
Zoro is the classic all out battle decided in one crude hit whoever is stronger wins.
Hell almost every fight took a lot of thinking. Fighting OZ, assaulting Enis Lobby and technically even luffy making gear second can be seen as the underdog trying anything to win. He see CP9s technics and the only way he could find a way to win was to essentially copy and adapt them for himself(gear second and third) otherwise there was no chance, and even with them they barely lost.
I mean I like naruto and bleach, read them weekly. They were awesome when they started out, became shit over time, kinda started getting better in last year or so. One Piece is so consistant and everything is new from arc to arc and Oda came up with a way that he can vary his arcs great deal without plot holes thanks to the fucked up weather in grand line. So he can change the setting and have it make sense, from winter islands to summer islands to thirller bark, enis lobby, the red line, the countless islands, screwed up weather, the log posts, fishman island. He mixes silly with fun, adventure with design and inventive beyond belief. That is what makes One Piece the greatest manga, not that it sold the most manga copies of any book let alone manga ever sold in Japan(which it did), and not because the fanbase seems much more down to earth, don't take things seriously and don't flip out or call other series down. It's because the person behind the magic knows what we like, and knows exactly how to tell a fun story. It is a series that it truly feels i'm along with the adventure. During Naruto I feel Naruto will always just pull the old nine tails card, but I have to admit some of my favorite fights are in naruto, and the begining up till episode 60 of bleach is amazing. Otherwise the shows feel empty to me, random plotkai.
See Oda can pull surprises from any angle, and you know eventually strawhats will win, but in some cases they lose... Sometimes they win... Sometimes they come out half alive, sometimes nearly dead. Sometimes they get utterlly destroied. It feels like a real story, when suspense comes it might actually be something bad happening. Hard to explain.