Don't know if I can pick a singular least favorite. I have entire artists I despise for their particular vocal styles.
Over-the-top wailers: Adele, Gotye
Pouty mumblers: Lana Del Ray, Billie Eilish
Billy goat bleeting: Stevie Nicks
Take your pick of their respective overplayed hits and mash them together. That's my most hated song.
Edit: forgot Billie Joe Armstrong of Green Day, sounds like he swallowed his tongue