So last summer after Kingsport, I made some rudementary system to guage how difficult monsters are. I took different things into account, like awareness, damage, difficulty of checks, movement type, toughness, etc. While certain things may have been weighted more so than others, I feel that this list is pretty close to my experiences.
Keep in mind that this is a list based (almost) purely on how difficult they are to kill in combat. Please, comment, criticize (constructively). If I can find or deduce the system I used I'll post it. Also, if you insist on completely disagreeing, please be polite about it, and make sure you say why.
Dhole (27)
Star Vampire (25)
Flying Polyp (24.66)
Werewolf (22)
Wizard Whateley (21)
Tcho-Tcho Priest (21.5)
Hound of Tindalos (21)
Hunting Horror (20.66)
Star Spawn (20.66)
Shoggoth (20.16)
Wraith (20)
Dark Young (19.16)
Gug (18.17)
Colour Out of Space (18)
Mummy (18)
Spectral Hunter (18)
Crawling One (18)
Gnoph-keh (17.5)
Leng Spider (17.17)
Vampire (17)
Formless Spawn (16.66)
Cthonian (16.33)
Ghost (16)
Moon Beast (15.66)
High Priest (15.66)
Serpent People (15)
Warlock (15)
Goat Spawn (14.66)
Yithian (14.33)
Ghast (14)
Elder Thing (13.66)
Rat Thing (13)
Skeleton (13)
Dimensional Shambler (13)
Shan (12.33)
Fire Vampire (12)
Witch (11)
Tcho-Tcho (11)
Night Gaunt (10.67)
Ghoul (10.66)
Mi-Go (10)
Byakhee (10)
Zombie (6)
Cultist (5)
Maniac (4.25)