I've completed all games, expect the Suvivor games. They were just too boring to complete.
Though it's a really tough question, since I'm a huge fan of the RE series. Though my two favourites are RE4 and RE2.
My reasons are because of the characters, the gameplay, the story and the atmosphere in the game. Especially in RE4, where they've added so many elements making it one of the best games that I have ever played.