My favorite's are the Hunters and the Regenerados.
The one that dissapointed me was the Licker's one, but I think that that's because I already knew about them before hand, and because I always found Resident Evil 2 extremely easy, they're not that hard to take out from the beginning, and they become a lot easier after you understand that they're blind. I always felt safe with my shotgun around them and I had little trouble avoiding them when I wanted to (for the most part I didn't need to, I did only when I aimed for Claire's special outfit).
Hunter's on the other hand... Are a beautiful pain in the a**.
I knew about them too before playing RE1 (I'm talking about the original, not the remake, they're easier there), but that didn't stop them from scaring the s*** outta me; they scare me shotgun or not, because even with it they're so hard to avoid and they can be fast in some instances, it doesn't help that RE1 can actually be hard and punishing with his lack of ammo. So yeah, I loved their introduction: it was terrifying, tense and made me feel unprepared even if I knew about it.
Talking about the Regenerados (and Iron Maiden) instead... Well, there isn't much to say, their very existence scares me lol
their introduction in RE4 was perfect, so much so that it still terrify me to this day. I didn't see that coming at all, I was totally unprepared. By that point of the game I felt like a boss, I felt like I had everything under control and then they appeared and gave me a run for my money (it didn't help that I didn't take the special scope during my first run).
Also, I love the way the game gives you hints about them way before you met them (it's nothing new, but I think that it was beautifully done here).