The reason I changed: because Quake's music is more moody and satisfying than DOOM's, and I like the gameplay and monsters much better.
And I say "series" is because I like all of them. I have no particular favorite. I like all of them.
DOOM will be ONE of my favorites, because it's a relative of Quake. But the Quake series is my new favorite game of all time.
And so, the Star Wars series is my favorite movie of all time. I have no particular favorite. I like all of them, special or non-special.