One Piece is the #1 Shonen in the world for many reasons, and one of them is the incredible range of characters Eiichiro Oda is able to think up and illustrate. There are pirates, marines, and third parties of all types and almost all of them are lovable in their own ways.
Part of the reason for this is that Oda likes to turn tropes on their head by making tough characters weak and weak characters tough which makes it fun to speculate how strong each newly introduced character will end up being. Let's take a look at some of the best examples of characters that look like they could be the antagonist of an arc on their own, but didn't manage to live up to that hype.
10 Marine Captain Axe-Hand Morgan
It only seemed right to start things off with one of the first villains introduced in the series. Axe-Hand Morgan is the viewers first look at a Marine that is designed to be individual, Morgan is a captain and basically the first real significant Marine to appear in the storyline. And, as such, it would make sense for him to be tough or put up any sort of fight for Luffy or Zoro, right? Wrong. The dudes metal jaw, Axe-Hand, and overwhelming stature purposely mislead the viewer with stereotypical "manly" design aesthetics so that when the dorky kid in flip flops beats him wholeheartedly, it's extra surprising.
9 Don "Foul Play" Krieg
Then comes Don Krieg, one of the best fights the Straw Hats have before entering the Grand Line. The battle against Don Krieg shows what happens when someone focuses too much on one aspect of being a pirate. Kreig really nailed building up a crew, so much so he had 50 different Pirate ships working underneath him. Problem is, he didn't focus enough on his own strength, his crews strength, his overall goals, or his research before entering the Grand Line. We don't see another pirate with 20+ ships underneath them until significantly further along in the story, so it was a bit weird to see someone so early with such a huge backing who didn't have the moxie or strength to back it up.
8 Smoker The White Hunter
The story of Smoker and his Journey from Loguetown to the New World is almost tragic, as the guy had such an imposing entrance that didn't persist the whole way through. This is the Marine that introduced the viewers/readers to Logia-type Devil Fruits, and he seemed basically unbeatable before Haki became a thing. Then, once the time-skip happened, he just keeps getting beaten down further and further and has no real hope of catching up to Straw Hat Luffy, the Pirate who made him leave him home in the first place. Whether its the Cigars, the scars, or his 6-pack abs, Smoker looks like he'll be tough, but he's not about to surpass Garp anytime soon.
7 The Saruyama Alliance Brothers
The Saruyama Brothers of Jaya Island are some incredibly interesting characters that are part of one of OP's best arcs and don't really get the spotlight their designs deserve. Both of them are "apparently" humans with ape-like features, and they shed light on an aspect of the One Piece world that doesn't get enough credit, sunken ship salvaging. Masira and Shoujou are both gigantic in different ways, Masira in his chest & arms and Shoujou with his belly and hair.
Both of them easily look like they could be strong opponents, but it turns out their specialty in salvaging doesn't quite translate to combat all too well since Pre-timeskip Bellamy had little trouble with them.
6 Ex-Warlord And Mastermind Of Baroque Works, Sir Crocodile
Now, for where he's encountered in the Grand Line, Crocodile is super tough. This is before Haki, The New World, or the Time-Skip, so the Straw Hat pirates are comparatively pretty weak. But, when looked at in comparison to his other Logia users, Crocodile is pretty dang weak. Logia's usually can only be harmed by Armament Haki, yet Crocodile can take physical damage if he becomes wet in any way. Plus, while being a Sand Man works amazingly in a Desert, outside of it there's not much Crocodile can do to enemies from a far distance. Meanwhile, Logia's like Kizaru (though he has his own issues) can shoot lasers across the entire ocean if they needed too. It's a shame too, cause Post-Timeskip Croco doesn't seem to be all too much stronger either.
5 The CP9 Assassin, Blueno
Blueno is the first member of CP9 (that isn't brand new) defeated by the Straw Hats, and he has arguably one of the best Devil Fruits of the bunch. Outside of Kalifa's Soap Fruit, the rest of them use Zoan type powers (which are also underrated) that all have the same weaknesses. Meanwhile, Blueno's Door Devil Fruit is a lot more conceptual and could've been way stronger if Blueno had thought about it more. Instead, he serves as a stepping stone for Luffy conquering the Rokushiki technique and is quickly dealt with to show off Luffy's Gear Second.
4 Bellamy The Hyena (Post Timeskip)
Bellamy the Hyena was briefly mentioned earlier but now its time to look into how this lad isn't as strong as his Devil Fruit should be. Most of the things Luffy does with his Gum-Gum Fruit mimic what Bellamy could do with his Spring Fruit, but he just doesn't. The hyper compression Gear Fourth uses could be done by Bellamy as well, but he doesn't. Both Pre and Post Timeskip, he simply uses his Fruit to jump around and it's a serious disappointment. Imagine if Bellamy saw Luffy as a Rival and both of them trained their bouncy powers in different ways? We could've had something as amazing as the Katakuri fight, but much earlier.
3 Captain Of the New Fish-Man Pirates, Hody Jones
The Fishman Island Arc is low down on most fans "tier-list of One Piece Arcs". It's all over the place, serves only to show off how strong the Straw Hats have become over those two years, and drags on for a while. Plus, Hody Jones is arguably one of the worst parts of it. He's a racist made racist by his environment, and while that's understandable, it's not forgivable or even remotely likable.
Hody Jones is a close-minded bigot who dopes up so much that he temporarily becomes incredibly powerful before turning into a husk of what he once was. Even if Luffy didn't beat him, Hody was going to turn skeletal soon, and that sort of ruins the Arc retroactively, doesn't it?
2 Ex-Flying Pirates Member, Wadatsumi
Speaking of Hody's New Fishman Pirates, the giant Wadatsumi is part of it. Wadatsumi is the only Fishman who managed to avoid the aging effects of the Energy Steroid, likely because he's gigantic and he only ate one. Still, this Tiger Blowfish Fishman looks like Blackbeard's Sanjuan Wolf (who's bounty is still unknown), and that's pretty scary. He can carry entire ships, buildings, and poneglyphs with little to no issue and defeat Krakens with a single punch. But, as it turns out, a lot of that strength is overexaggerated or "puffed-up" and this dude can easily be brought down with a single strong kick to his vitals like Sanji does during their confrontation.
1 The Germa King Named Vinsmoke Judge
And lastly, while on the topic of Sanji, let's talk about his Father, Vinsmoke Judge is the King of the Germa Kingdom, the leader of the Germa 66, and an incredibly intelligent scientist. The Germa Kingdom is known for its powerful weapons of science, and Judge looks like he's completely ready to take on anyone at any time with his Raid Suit. But, without it, Judge is just a normal man, and he's easily beaten by Big Moms devious plan to take over the Germa Kingdom. Luckily Sanji saves him, but it immediately shows how weak Judge would be without his precious inventions. Plus, the dude knows everything about Haki, yet we don't see him really using it ever.
