BM's introduction in Fishman Island was really terrifying, and her proper introduction being godzilla had this cartoony vibe was also pretty amazing. Her background history chapters are some of the best post-TS, a "natural born monster" and highly sought out by underground Marine forces as well. Her feats IMO were on par for Yonko of her type, a tanky fighter who's on the slower side but compensating with a Homie that she uses for mobility, and crazy raw strength that the MCs early on had no choice but to dodge because otherwise they would be one-shot by them. A powerful DF that is extremely versatile and does many things from giving her flight to healing her and making allies out of her surroundings. And her fight with Kaido for 3 days/nights solidified her high top tier status, in addition to having ACoA + ACoC just for completion (though that was never her main fighting style). Her goals to find the OP and to make a world where every race can live together have been pretty clear-cut. She was an unpredictable character and her charisma was great, although she had her extremely questionable moments like Mother Mode and O-Lin.
Kaido's introduction was also really impressive, but weird. Wanting to suicide by dropping off from the sky was based af. His proper intro as a drunk dragon had this cartoony vibe to it as well. His background was completely rushed and cramped and forced in the worst imaginable time when he was literally a punch away from getting beaten. Arguably his greatest flashbacks were those from King, where he was portrayed as King's savior and their mentor-savior like relationship. His goals were so muddled and convoluted I still don't know to this day what was the point in all of his suicide bullshit. His title as the "World's Strongest Creature" is incredibly justified and being a complete fighter with all 3 Haki mastery types and quick succession of transformations in between. An endurance monster, worthy of his 1v1 reputation. Overall, a decent character for a villain whose presence is imposing because felt like nobody could win against him until Joyboy was pulled out of Luffy's ass. But maybe even that is a flaw written for his character if that required such a thing.