Every Showa non-Godzilla film is like a little gem, I love to get lost in the story-telling, the look of the movie, its unrivalled (anywhere or anytime) color/cinematography, the widescreen compositions, the music.... some shine brighter than others, and I'm pretty much in line with the what seems to be the general opinion here..... Frankenstein, Gargantuas, Mothra, Rodan, Mysterians, Gorath.....but I do love them all.
I would also arbitrarily add 1973's Submersion of Japan to the list, even though I have only ever seen the 1974 abortion Tidal Wave. What remained of the Japanese footage though, was pure Toho Showa, albeit early 70's (Japan's depression?) .....I'm currently trying to track it down.
I had a difficult time out the gate with many of the movies, such as King Kong Escapes....like others here, I was enchanted by it as a kid, in spite of my dad and brothers heckling the movie and me for watching it. Then that time came when I tried walking away from these movies, and now I just embrace the damn things like crazy.
I even got my ex's nephew into them, and he's 23 now, and still loves them.