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. Hedorah aka Godzilla Vs the Smog Monster

The 70's brings a new era to the Godzilla franchise, the superhero / pro-wrestling era where Godzilla is the protector of Japan / Earth and fights daikaiju that threaten the peace. "Godzilla Vs. Hedorah" aka "Godzilla Vs. the Smog Monster" is also the biggest message movie since the original and the director gives the movie a independent / art film feel which fans of the franchise either like or hate. It also has the most screen time for Godzilla and his opponent double that of almost all previous films.