If you loved Great Mazinger vs. Getter Robo, try Great Mazinger vs. Getter Robo G: The Great Space Encounter
A bridge between a film you've already seen and one most people haven't. Here's what they share, and what the second one does that the first one doesn't.

Great Mazinger vs. Getter Robo

Great Mazinger vs. Getter Robo G: The Great Space Encounter
What they share
Both films are directed by Masayuki Akehi, and they both carry the playful mood tag, and they sit in Action / Animation / Science Fiction territory. If that's the register that drew you to Great Mazinger vs. Getter Robo, the second film will land in a comparable space — through a different lens.
What Great Mazinger vs. Getter Robo G: The Great Space Encounter is
You pilot Getter Robo against alien invaders wrecking Japan. But their UFO and monsters overpower you. A teammate dies. Great Mazinger steps up. Yet even that robot struggles against such might. The crossover's brisk pacing is a toyetic template. It leaves one wondering what other combinations could work.