Godzilla Vs. Mechagodzilla aka Godzilla Vs. the Bionic Monster aka Godzilla Vs. the Cosmic Monster

For Godzilla's 20th anniversary Toho wanted a new opponent, something he had not faced before. Remembering they had Kong face a robot duplicate, they decided to have Godzilla face his own mechanical doppelgänger. Also made the aliens controlling Mechagodzilla, in honor of King Kong, are apes, or maybe it was because of the popularity of the Planet of the Apes movies.  
      

Godzilla Vs Megalon

The script for this movie was originally just Jet Jaguar Vs. Megalon, but Toho thought twice and figured it would do better with Godzilla and Gigan added to the fight card. Also like the last two movies Godzilla battles at night till dawn, however it is easier to see the nighttime action then the last two.

Godzilla Vs. Gigan aka Godzilla on Monster Island

"Godzilla Vs. Gigan" starts the heavy use of recycled effect footage. Godzilla ends up with a lot of screen time, however a lot of that is stock footage or spent swimming. This movie was also a later release in the US not coming out till 1977 five years after it came out in Japan. And called "Godzilla on Monster Island" even though he spends very little time on said monster island.

Son of Godzilla

Godzilla's south seas vacation continues and we are introduced to Minira or Minilla the small grey pudgy daikaiju nobody asked for.

Ebirah, Horror of the Deep aka Godzilla Vs. the Sea Monster

Taking a break from the city destruction and world saving Godzilla takes a south seas vacation in a film originally written for King Kong. We also have a new director Jun Fukuda, Sadamasa Arikawa taking a more active role in effects, and Masaru Sato does the score.