I love reading lists like this. Like everyone else, it is really tough to limit it to 20, I could probably go to 40 or 50 as well. My favorites tend to stay the same from when I was a kid, add in many of today's Superhero movies (or Tolkien-related), and a lot of animated stuff. I have a lot of good memories going to animated movies the past few years with our daughter. Here's mine off the top of my head, in no particular order:
1. Star Wars
2. Empire Strikes Back
3. Return of the Jedi
4. Raiders of the Lost Ark
5. Indiana Jones and the Temple of Doom
6. Indiana Jones and the Last Crusade
7. LOTR: The Fellowship of the Ring
8. LOTR: The Two Towers
9. LOTR: Return of the King
10. E.T.
11. The Goonies
12. Guardians of the Galaxy
13. The Avengers
14. Ghostbusters
15. Harry Potter (any of them really, Azkaban is probably my favorite)
16. The Incredibles
17. Captain America: The First Avenger/Winter Soldier
18. Spider-Man 2
19. Scott Pilgrim vs. the World
20. The Muppets
Others: Karate Kid, The Hobbit, Toy Story Trilogy, Almost All Pixar, Nolan's Batman Trilogy, Back to the Future, Jurassic Park, Pirates of the Caribbean: The Curse of the Black Pearl, Iron Man, X2: X-Men United, Despicable Me 1&2, the Iron Giant, Christmas Vacation, Caddyshack, Uncle Buck, Planes, Trains, and Automobiles, Chronicles of Narnia: the Lion, the Witch, and the Wardrobe