These numbers are from Wikizilla.
Godzilla 9,610,000
Godzilla Raids Again 8,340,000
King Kong vs. Godzilla 11,200,000
Mothra vs. Godzilla 3,510,000
Ghidorah, the Three-Headed Monster 4,320,000
Invasion of Astro-Monster 3,780,000
Ebirah, Horror of the Deep 3,450,000
Son of Godzilla 2,480,000
Destroy All Monsters 2,580,000
All Monsters Attack 1,480,000
Godzilla vs. Hedorah 1,740,000
Godzilla vs. Gigan 1,780,000
Godzilla vs. Megalon 980,000
Godzilla vs. MechaGodzilla 1,330,000
Terror of MechaGodzilla 970,000
Why the sharp decline after 1966?