The Nemesis was by far the creepiest boss in the series. Like the thread starter mentioned, the music changed when he was on his way. This was extremely scary in my opinion. I used to hate it when the music kicked in, especially when I was running low on ammo.
He was just so damn terrifying. I hated it when I had to fight him. That's why whenever it gave you the option to fight or run, I almost always clicked run. Nothing could make me stop and fight the beast.
Still, despite being creepy, he was pretty cool.