RE2 was cool/is my favorite old school RE, but it isn't scary at all. RE3 is one of the scariest games I've ever played. Not because it's actually scary, but because you know that at any moment, Nemesis might break through a wall and kill you in two seconds. And even if you run away, he will follow you until you are destroyed. Code Veronica was cool too, but Steve pissed me off so much that I have to hate it. And yes, I know what happens to him. He still pisses me off and I hate him.
RE4 is by far my favorite. It's one of the few games where, upon completely beating every aspect of the game (on multiple difficulties, after unlocking EVERYTHING etc.) I still wanted to play more. And ever so often I still go back and play Mercenaries. Plus, let's face it, Leon S. Kennedy is a billion times cooler than any of the other main characters in any of the other REs. The only one who comes close is Chris Redfield, and even he's still kind of lame. The only reason why he's cool at all is because he can ALMOST beat Wesker in a fight, which is pretty hard to do considering how awesome Wesker is.