The Grunt has always been my favourite, it still has human features; for example, a face, as hideous as that face is I'll never forget my first encounter, how scared I was, mainly because with recognizable human features like a face it displays more emotion. To see that face again during my first playthrough wasn't only terrifying but it was also - in a way - devastating and sickening, I just loved how deeply terrified I was of them.
The Brute was very intense but hardly as as scary and traumatizing as the grunt for me.
The Kaernk was quite unexpected but never really deeply scared me; although, it made me kind of scared of water.