Hopefully, one that hasn't been used to death. I thinking the following would be adequate:
Titanosaurus: Also a huge acuatic reptile, useful to represent Godzilla's oposite side. Because he doesn't use beam or projectile weapons, his battle with Godzilla can be more carefuly crafted than the usual array of mystical mumbo jumbo and deus ex machinas we've been seeing for decades.
Biollante: Another that can be Godzilla's opposite. The origin story would need to be changed to something more sophisticated, audiences today can't simply swallow "I've mixed Godzilla's DNA with a rose!". This where an interesting comment on bioweapons and society's fear of them comes in handy.
Spacegodzilla: In his debut film, Spacegodzilla suffered from poor effects and a lazy origin story. In this era of photorealistic CGI and rising young directors, a lot can be done to bring what was just "Oh that's kind of cool" to "Oh that's Godzilla's coolest enemy yet". Spacegodzilla is also another one that can mirror Godzilla and become an oposite of him.
Ebirah: I just like my simple, non power ladden, enemies in the series. What is Ebirah besides a giant shrimp? Not much, but this is part of his natural charm. If the script is not just a King Kong movie with Godzilla replacing the ape, then so can Ebirah be updated to become a sadistic menace (and Godzilla the monster we root for).
King Ghidorah: Dragon or ogre?