Mecha vs Kaiju (True20)
Worlds of Imagination & Adventure Await You!
With True20 Adventure Roleplaying you embark on journeys of the imagination, with heroes and stories you and your friends create. This book tells you everything you need to know to play an exciting adventure roleplaying game: how to create your own heroes, how to handle action and conflict, how to narrate adventures, and how to keep it all fair and fun.
True20: The Original Mecha vs. Kaiju Rules [ edit ]
True20 was the first system used for Mecha vs. Kaiju, and may be appealing for this reason. Another reason is that True20 is based on the Mutants and Masterminds rules, which in turn were based on the "D20" rules (ie. Dungeons and Dragons, 3.0/3.5 Editions). As such, fans of that edition of Dungeons and Dragons, or of Mutants and Masterminds, my find these rules the easiest to learn.
The system is also very popular: on DriveThruRPG it has a 4.5 star rating from 47 reviews (at the time of writing)! It's RPG Geek score is a bit more mixed however, with a score of 6.29/10 from 42 reviews. However, if you look a the details of those reviews, you generally won't find any specific criticisms, except for one that says it's "Lightweight in all the wrong places, but" that they "actually prefer this to the regular d20 system."
Ultimately, if you're looking for "D20 Mecha vs. Kaiju", True20 will be a great option, but for a more modern take on the game you'll want to look at the Fate or Cypher versions.