Have to be a tie between Verdugo and Tyrant-103 from RESIDENT EVIL 2. Verdugo was an awesome stealth boss, and Tyrant 103 was just ruthless and unforgiving.

Mine would be the big monsters from re4&5 those are my fave boss battle

Try putting these all into ONE post next time, k?
1st link: = "El Gigante"
2nd link = "El Lago"
3rd link:= "Executionor"
(try taking a peak at the monster database here: http://www.biohaze.com/enemies.html so you know the name of the enemies, or in this case, favorite bosses)

This is a hard one for me. I think the Tyrant battles from the original are my favorite though. Just because I didn't expect them nor did I expect to be able to blow it to pieces with a Rocket Launcher.

I also really like the Krauser battle from RE4.

The executioner is actually called kifu in the data base sorry i didn't know what the name was of the others really

And urobrose wesker to so ive got to go for now get back to some stuff

For me (since I hardly remember the old games, I was young and didn't play very far through - mostly watched my brother)
it would have to be a toss up between Krauser (RE 4), Wesker (RE 5) and Salazar (RE 4 - mostly because I enjoyed killing the annoying bastard :P). I remember running from Nemesis (or my brother running from him) in one of the games as Jill for like a whole hour for fun, we voiced it over with Nemesis going "Come back baby. Let's hug". I'm sure he had a rocket launcher...