My top 31 are:

1. The Thing (1982).
2. Aliens
3. Alien
4. The Deadly Spawn (a.k.a. Return of the Aliens).
5. Re-Animator
6. Phantasm
7. Event Horizon
8. Videodrome
9. Scanners
10. The Fly (1986)
11. Them!
12. The Fly (1958)
13. The Blob (1988)
14. Day of the Dead
15. Slither
16. The Blob (1958)
17. Halloween III: Season of the Witch
18. Killer Klowns from Outer Space
19. Bad Taste
20. They Live
21. Critters
22. Critters 2 The Main Course
23. Night of the Creeps
24. The Thing from Another World
25. The Return of the Living Dead
26. Lifeforce
27. The Faculty
28. Evilspeak
29. The Mole People
30. Species
31. Let Sleeping Corpses Lie (a.k.a. Living Dead at The Manchester Morgue
32. Doom
33. From Beyond
34. Invasion of the Body Snatchers (1978)
35. Invasion of the Body Snatchers (1957)
36. The Crazies
37. Night of the Comet.