My favorite games will always be the classics but the bosses that take the cake for me are from Resident Evil 4, specifically Verdugo and U-3 aka "It", i've always considered this 2 encounters perfect bosses the first time you fight them, they where the ones that felt the most dreaded, threatening and bad ass of all RE for me.
As for disappointment, while i loved fighting Jack Baker, hes regular form goes out waaay too early and i didn't liked the last fight with him transformed having to shoot areas around his body, the mechanics of the fight where more anoying than anything and pretty archaic for the times we live in.
I think the one I remember most was Nemesis from RE3. He was insanely hard to kill on normal mode so I always ran away whenever he showed up and hearing him yell "STAAAAARSSS!!!" was always creepy along with that music that played when he's looking for you.
I think the one that disapointed me most was Lisa Trevor. While I do like her disturbing appearance and tragic backstory I found her to be too easy to deal with even in the final boss fight with her.
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).
My first favorite is Nemesis because he came out of no where and killed that P&%%y Brad Vickers all in a span of about 30 seconds, plus the fact that until the very end no matter how many rounds you pumped into him he'd be down but never out for long, not to mention he'd follow you from room to room relentlessly which i give the game's creators points for and I wish they would do for the other lesser monsters as well for a realistic touch.
The other favorite had it not been created WOULD have been the hunter but I give the nod to the Iron Maiden, everything about this creature from its breathing to the way it walks, attacks, regenerates body parts scares me. Id love to see it make a return someday and even tangle with another deadly B.O.W.
Dissapointment wise would have to go to the tunnel snake in Code Veronica just because it was a redone version of the digger from Re 3, and the alligator from Resident Evil 2 because it felt like the easiest "boss fight" if you can call it that in the franchise