No. 5: The Wendigo

Credit: Marvel Comics
First appearing in The Incredible Hulk #162 in April 1973, the entity known as the Wendigo is a spirit roaming the Canadian wilderness. When a human cannibalizes another this spirit possesses the human and transforms them into the man-eating beast men call THE WENDIGO!
Possessing greater strength, stamina, and durability than even the Hulk and an insatiable appetite for human flesh, the Wendigo is a terrifying monster the likes of which few who see it survive to tell the tale.
The Hulk is one of the few who can tackle the Wendigo alone and even then he sometimes needs help from Wolverine or Captain Marvel (Genis Vell) to get the job done. If anyone ever had doubts as to whether the Hulk was a hero or a monster, one look at the Wendigo and you'd have your answer. A classic Hulk villain steeped in actual Native American myth, the Wendigo is one of Hulk's most formidable foes.
The Wendigo curse was even transferred to the Hulk at one time creating a hybrid Hulk/Wendigo monster!