Most unpopular among my friends would be something along the lines of "metal is the best music" or "electronic music/techno sux, because it's all the same".
I like to troll a bit with them, but honestly I do like me some metal. It's so variable, there's progressive Opeth on one side, brutal machinegun Cannibal Corpse on the other, insane meatgrinder Mayhem on the third and groovy jumpy Pantera on fourth. And we're only at the beginning. But I get that some (most) people find it all too heavy. Their loss, not mine ;-)