Any monster that can turn you into a monster is pretty bad, but the worst are things like lycanthropes, who do it without actually killing you. I mean, what is even the point here? "Oh sorry Jane, you're a werewolf now, that means you're an NPC because otherwise you'd be overpowered for having the nerve to get bit by a werewolf...".
Actually, on the topic of werethings, creatures that are only scary if you have no idea what they are/not prepared to fight one. There's this whole metagame thing about "you don't know troll regeneration can be stopped by fire" that makes my eye twitch every time it comes up.
Things aren't so bad these days, but back when werewolves were either a cakewalk or a TPK based on whether or not you had silver weapons always made me wonder what they were for- either you can fight it and it's a joke, or you can't and it infects the whole party?
Low CR monsters that can murder unprepared adventurers in general. 5e has made some things less obnoxious (like ghouls, thank Gygax), but the Shadow is still able to obliterate anything with an AC of 16 or less. Only 16 hp? Look again, they are at least resistant to just about anything you can hit them with! And if you're Strength-based melee, one hit can render you into a helpless bag of hp, unable to even wear your heavy armor! Not that they care about hp, because they can kill you even if you have a thousand hit points!