Curmudgeon, perhaps. Or at least curmudgeonly.

Or choose from something here: https://www.thesaurus.com/browse/curmudgeon

Among those, "crank" stands out. This is usually someone who holds a (often provably) wrong, unconventional or unpopular opinion and insists that their way is the right way and that everyone else is wrong.

Cranks can be right, which a crank will hold onto dearly and use in their arguments, even if the considerable majority of cranks turn out to have been wrong.

On the other hand, cranks tend to beat their metaphorical drum a lot more than someone who simply refuses to pick a side and keeps their trap shut.