A tricky question. But my vote goes for PH. I don't think that gaming world ever has seen such a charismatic and intimidating foe with such brilliant story. Plus I still get the chills everytime I first run into him in that long corridor at Woodside. Creepy! He just stands there staring(?) at you and you just know that you haven't seen the last of this guy.
Valtiel just makes me feel uneasy and insecure. Cool character nonetheless
By the way, sorry for the off-topic but does anyone know what the word "Valtiel" means? Or is it something Konami just came up with. I tried to google it but all I found were SH-sites or people who strangely enough do SH-fanart