I'm surprised Tashigi hasn't come up, I really hate how Oda handled her at Punk Hazard but I'm glad Smoker's humiliation was mentioned here.
Big Mom getting literally rolled by Robin as much as it was a massive and much needed Win for Robin. Kaido crying over his smile factory.
Kinemon's fart talking.
Kiku vs Kanjuro.
Shinobu in the present storyline.
Machvise as a character/design and him being a copycat of Miss Valentine with her DF and fighting style.
Orochi's "return" and both his "deaths", as well as his personality and design in general.
Also Orochi's "elite ninjas and samurai".
That sumo guy who was perving on Kiku and got beaten by Luffy too.
The gifters are probably the most humiliating aspect of One Piece though. Especially the likes of Hamlet and Four Tricks for example, and that one hippo one.
Oh wait I might know the best one? A Yonko's army made up of failed artificial Zoans being literally mind controlled by a little girl and her extremely plot convenient devil fruit. Maybe a Deus Ex Machina even?
A mere child is drastically turning the tables in a war all thanks to her DF. Turning a Yonko's soldiers to her side and under her control.
A Yonko, some of the strongest, most powerful and feared pirates and characters in the entire One Piece world. Sigh.
Imagine if Oda actually ever clowned the Admirals like this? Not that it would ever happen...