Godzilla vs. The Smog Monster is true high point of the series. It is just so bizarre that is stands out among the rest in a big way.
Unfortunately, both Godzilla vs. Gigan and Godzilla vs. Megalon have good ideas, but lack the production values to really do a whole lot with them. Godzilla vs. Megalon is my pick for worst film in the entire series. Godzilla vs. Gigan is still good, even with its obvious faults.
Godzilla vs. Mechagodzilla is awesome. It captures some of the wackiness of Godzilla vs. The Smog Monster, but also adding some great elements to the series. The SFX took a turn for the better here as well as Mechagodzilla was wonderfully constructed. I also loved how they made Godzilla bleed like crazy throughout his encounters with Mechagodzilla. The sight of MG flying through the air blasting Godzilla as blood flies everywhere is one of those memorable visuals that just stuck with me through childhood.
However, my favorite G 70s film is Terror of Mechagodzilla. Honda return to the director's chair was a welcomed presence as he took the superhero Godzilla and gave it a serious tone that took Godzilla out of kiddie territory just a little bit. I also loved the ideas of the cyborg girl, the mad scientist, Titannosauras, and of course those aliens came back as well. The climatic battle is one of the best in the series as Titannosauras and Mechagodzilla totally dominate Godzilla, and we are led to believe that he is finished, only for him to pop up out of the ground and start whooping some ass. Its actually one of my more favorite G-Films as it combined the best elements of the 60s films with the best elemnets of the 70s films, and it worked quite well for me.