How much you wanna bet many of us will be updating our lists very soon?
#1: The Return of Godzilla (1984)
My first and favorite Godzilla movie. There's so much I love about this film. Even though it was made decades ago, it still has cultural significance today.
#2: Godzilla vs. Biollante (1989)
As of right now, I believe this to be the definitive Godzilla representation. It's easily my favorite Godzilla design. Biollante, too, is one of the most unique, frightening monsters I've ever seen. Great storyline, memorable characters, and plenty of action to keep us coming back for more.
#3: Invasion of Astro-Monster (1965)
Classic. I remember watching this film so many times as a kid. It plays with so many science fiction elements, it has inspired many of my personal projects.
#4: Godzilla vs. Destoroyah (1995)
Nowadays, beloved characters are getting killed off left and right, to mixed reception. How Godzilla's life, death, and legacy was depicted in this film is perfect. Absolutely stunning in every sense of the word. It'll easily become a classic in later years, if it hasn't become one already.
#5: Godzilla, Mothra & King Ghidorah: Giant Monsters All Out Attack (2001)
A nice new take on the Godzilla mythology. It continues to be one of the few Millennium films I can watch all the way through.
Honorable Mentions: Mothra vs. Godzilla (1964), Ghidorah, the Three-Headed Monster (1964), Ebirah, Horror of the Deep (1966), Godzilla vs. MechaGodzilla II (1993), Son of Godzilla (1967), Godzilla Against MechaGodzilla (2002)