Firstly, let me make it clear that the 5-star rating is for 'Godzilla vs Destoroyah' only.
'Godzilla vs Spacegodzilla' is really not very good and may give credence to the assertion that Toho was running out of ideas and for that reason was preparing to kill off Godzilla. The movie starts off reasonably well, with Little Godzilla being seized and held captive by Spacegodzilla, but the ensuing battle with Godzilla is ruined by too many action sequences involving the military.
'Godzilla vs Destoroyah' is undoubtedly the most moving of the entire series and rightly so, since this is the movie in which Godzilla dies. Birth Island where Godzilla lives is found to have been destroyed and the nuclear energy in Godzilla's body has reached critical mass. Very soon he will reach meltdown and explode, taking the whole of the Earth with him.
At the same time, a monster called Destoroyah has appeared which is a living oxygen destroyer. The authorities hope that by having Godzilla fight it, he may be killed without meltdown taking place. This is the lead-up to Godzilla's dramatic demise and the final scenes are probably the most moving I have ever witnessed. Akira Ifukube's superb music score pulls no punches and the requiem for Godzilla as the monster slowly perishes is enough to bring tears to your eyes.
Why do I feel such pity for a giant fire-breathing reptile which has laid waste to cities so many times and probably killed thousands of people in the process? Well, in reality I wouldn't, of course. But there is something rather sad about this creature who stands alone in the world and whose life is one long battle for survival against other creatures as well as mankind. Or maybe I'm too much of a sentimentalist. Whatever the reason, I wish Toho hadn't killed him.