Wolves. Period. No other enemy attacks in such large groups, and on death march, even a lvl 5 wolf pack can kill you, as they leave no room for dodging
Humans
PS: I hate Drowners the most, since they became with TW2 onward so supernaturally fast for undeads... with leap attacks like frogs..seriously, super annoying when they swarm you and attack you with their way too fast attacks from behind and easily kill you on Death March with a few attacks, if you aren't careful, making it basically essential agaisnt them to have permanently QUEN active or to have good dodge reflexes.
"Drowners are scoundrels who ended their wicked lives in the water. Drowned alive or thrown into deep water after death, they turn into vengeful creatures which stalk the inhabitants of coastal settlements." - BestiaryCandles.
Drowners are not undeads!
"Drowners are scoundrels who ended their wicked lives in the water. Drowned alive or thrown into deep water after death, they turn into vengeful creatures which stalk the inhabitants of coastal settlements." - Bestiary
Suggesting 'scoundrels' referring to human villains, they are in fact undead...
As for me, I can't stand drowners in huge packs, they all attack at the same time which is taking my health away in an instance on Death March.
I developed a passionate hatred for Sirens when going to all the undiscovered locations on my boat in Skellige.