To kick things off:
Game:
Favorite
Least Favorite
Male Playable Character:
Favorite
Least Favorite
Female Playable Character:
Favorite
Least Favorite
Normal Enemy:
Favorite
Least Favorite
Boss:
Favorite
Least Favorite
Tyrant:
Favorite
Least Favorite
Villain:
Favorite
Least Favorite
Location:
Favorite
Least Favorite
Favorite Music/Soundtrack
For my choices...
Game:
Favorite - Resident Evil 0
Least Favorite - Code Veronica
Male Playable Character:
Favorite - Billy Coen
Least Favorite - Carlos if we count support characters
Female Playable Character:
Favorite - Rebecca Chambers. The amount of work they did to redeem her awful RE1 character with RE0 is astounding.
Least Favorite - Jill Valentine
Normal Enemy:
Favorite - Hunter
Least Favorite - Spiders. They are really dumb and no threat.
Boss:
Favorite - G-Birkin, specifically second or third form
Least Favorite - Giant Bat in REZero. Really lame. Giant Scorpions are scary and the giant centipede had cool music at least.
Tyrant:
Favorite - Proto-Tyrant in 0 or Mr. X in 2
Least Favorite - The one in Code Veronica
Villain:
Favorite - William Birkin
Least Favorite - Albert "Matrix Reject" Wesker
Location:
Favorite - Spencer Mansion or Umbrella Training Facility
Least Favorite - Island in RE4
Favorite Music/Soundtrack - I think RE2 had the best overall OST.
However I'll pick a few favorites from my favorites. (If you are going to link vids, please put them in spoilers like I'm doing)
Resident Evil 2 - Second Malformation of G
Possibly the most iconic piece ever to come from an RE game
(SPOILER)
Resident Evil 0 - Save Music
The BEST save room theme.
(SPOILER)
Resident Evil 4 - "Path To Closure"
The Krauser fight music. I love the tension of the first stage of the fight with him so much and this music is a huge part of it.
(SPOILER)