You have to give the Japanese a lot of credit. As the Godzilla series moved along, they seemed to realize pretty early on that there aren't a lot of ways to improve on the concept of "giant, radioactive monsters that stomp on cities and fight other giant, radioactive monsters." In fact, it took them a full 20 years to stumble upon a suitable conceptual upgrade. And that was giant robot doppelgangers of giant, radioactive monsters. Enter my favorite Godzilla foe of all, MechaGodzilla:
And, of course, it worked so well that they brought it back for the second series of films*:
And the third.
MechaGodzilla - two great things that go well together, truly the Reese's Peanut Butter Cup of the kaiju world.
*Well, technically they brought up the concept three times in the "Heisei" series, but Mecha-King Ghidorah was just the original King Ghidorah with a cyborg upgrade, so he doesn't quite count, and M.O.G.U.E.R.A. was just plain lame. Demand the genuine article, people.