Who do you think was the better character?
God.... Kaido is effectively Jiren with a few quirks. Linlin's character has actual mental retardation built into her alongside being all over the place.
It's so far in the dirt, whoever's the winner isn't saying much.
Which of the two was the better villain? Kaido during the Wano arc or Big Mom during the WCI arc?
Katakuri was the villain of WCI, Linlin was a plot device. So automatically Kaido but he'd be underwhelming for a relatively short arc like Skypeia, let alone OP's biggest arc ever and antagonist to a 400 chapter saga.
Who had the better backstory?
Linlin, even if it doesn't benefit her character to be tricked and lied to her whole childhood life. Since it never comes up ever again.
In terms of what?
In terms of strength, Kaido.
In terms of writing, Linlin.
Jack got nothing all Wano, Queen got some decent development and King didn't get spotlight on him until his last 15 chapters.
The Tobi Roppo were largely introduced and defeated within the span of 20 chapters, in a climate that really didn't let us get to know them.
In contrast, Tottoland chewed the fat with the Charlotte Family and we got to see much more of their family dynamic and the way the larger crew operates around them.
Which fight do you think was more better: Luffy vs Kaido or Kid and Law vs Big Mom?
Luffy versus Kaido is easily better. Kid and Law v. Linlin is not worth the reread.
And finally who had the better conclusion during their defeat?
Linlin's was better executed, Kaido's on paper is better though.
More finality in his defeat, especially with how Oda made sure to include the King/Kaido conversation in #1036 and did a call back to that during Kaido's defeat. Kaido also went down one chapter after Orochi, so it was a bit more thematically solid.