MovieWeb.com ranked the 12 best monster transformations in movies. It's worth mentioning only ONE of these movies came out after 1990, which goes to show that practical effects stand the test of time.
Here they are:
1. "The Thing" (1982)
2. "The Fly" (1986)
3. "An American Werewolf in London" (1981)
4. "The Toxic Avenger" (1984)
5. "Cat People" (1982)
6. "The Wolf Man" (1941)
7. "Dr. Jekyll and Mr. Hyde" (1932)
8. "Frankenstein" (1931)
9. "Tokyo Gore Police" (2008)
10. "The Witches" (1990)
11. "A Nightmare on Elm Street 2" (1985)
12. "The Howling" (1981)