I didnt find the SH monster to creepy the only ones that really creeped me out were they zombie like people in SH4TR. The ones that float and fly at you and if you shoot them till they drop they just get back up and chase you some more.
But most of they RE creatures are freaky and creepy. LIke the Lickers, Giant Spiders, Cerberuses, MR.X, Hunters, most bosses, the Aligator, the Shark Neptune, Lisa Trevor (kind of freaky when part of monsters name is your own name), blind men, Mendeze (Big Cheese lol), Iron maiden, Graboid like worm from Code Veronica X, all tyrant prototypes in general.
All these creatures i think are freaky and scared me at times, some of them just the looks creeped me out. But overall id say that RE has scaryier, and SH has more sadistic creatures. But both of the series are good and both are same type of games: horror/third person shooter/puzzle genre.
But if you want to go with movies silent hill was by far the more creepy of the two series. I mean the only thing creepy abotu the RE series would be when somthing randomly jumps out of nowhere. Silent hill movie has barb wire lady. And hellbaby's.