The one you need to watch out for most is Flowey. http://undertale.wikia.com/wiki/File:Flowey_Talk_normal.gif
It's not so funny when you see THIS face. http://t3.gstatic.com/images?q=tbn:ANd9GcTMLrTjNTpnAv5LW0XBqlGqxnpzmulInZGnV55zPKdqo32XlCXquOMas1Q
jeff the killer is creepy but not scary enough, imagine a game based on jeff the killer? I'd play that