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)