Mothra vs. Godzilla (1964)

Forget that rubbish King Kong entry. M vs G is how to do a crossover!
There are two main villains, the angry Godzilla and a greedy businessman, but only one of them gets to wear a rubber suit in public.
It takes place mostly outside the city in an open landscape, so the sets are more expansive and less claustrophobic. That extra room gives Mothra space to swoop and fly around in. I really like Mothra's imago form. She’s colourful, noble, fights to the bitter end and has her own cool theme song.
It contains a similar kind of explosive imagination and charm as Ray Harryhausen's Sinbad films, but in a very, very Japanese way.
There's no doubt that the first film had more socio-political depth, but for pure awesomeness in the Shōwa era I go with Mothra vs. Godzilla.

3½ super-conductive metal nets out of 5