I often read that the Sicilian Sveshnikov is "the most controversial of all chess openings", then I read the same for the Sicilian Poisoned Pawn... and then same for the Botvinnik Variation of Semi-Slav. What is the most controversial opening or variation?
There's no quantifiable answer to this. In my subjective experience, the most controversial openings would be the Dutch Defense and the King's Indian Defense.
I have heard strong players (Sam Shankland, multiple times, including in some of his chess.com videos) heavily disparage the Dutch, but it is still defended occasionally at the top level, most prominently by Nakamura.
The KID is a more frequent guest in super-GM play, and Vladimir Kramnik has a low opinion of it.
At a meta-controversial level, the Berlin Defense causes some debate due to its drawish tendencies and popularity in top-level round robin events.