Godzilla's single most powerful foe

Who is it?

Destoroyah.

Why?

Godzilla's most powerful form, critical mass, was knocked down three times, dragged by his neck twice, and thrown entirely off the ground once before the fight ended and only managed to knock Destoroyah down once throughout the entire fight. Also, Destoroyah took not one, not two, but nine total blasts from Godzilla's red-spiral ray and sustained absolutely zero damaged from the first two. The second two managed to get the best of him when Godzilla exposed his belly flower thing, but after he broke apart and reformed the fight continued with Destoroyah as good as new. A further five blasts were used just to make him retreat from the area as Godzilla melted down, and even then Godzilla didn't deliver the finishing blow.

The military's freezing weapons and the Super X III were required to defeat Destoroyah.

Ultimately, Godzilla failed.

Many of Godzilla's other big time enemies are in competition with Destoroyah for this title, but the facts remain in the paragraph above and no other enemy can boast such a resume against Godzilla in a single engagement.

Spacegodzilla retreated after about three to four blasts. Mechagodzilla II was destroyed outright in three to four direct blasts. The red-spiral ray wasn't even required for King Ghidorah or Mecha-King Ghidorah, nor any other enemy in the Heisei series. Kaizer Ghidorah only took one red-spiral ray from the mutant's boost of power, but even then it only took one blast.

Again, Godzilla's single most powerful opponent is in fact Destoroyah. There are no other enemies which can overcome that fact. Godzilla's critical mass state is his single most powerful form, and another fact is that he only got stronger as the fight went on because his radiation levels continued to rise with his temperature. This is especually true at the end of the fight when his body really started going out of control, and even then it was not enough to defeat Destoroyah outright.

The video games are non-canon to the film franchise, and they take several liberties with the monsters and their abilities versus what we see on screen in the films where the canon is more or less absolute.

Godzilla's most popular opponent?

King Ghidorah, hands down.

Why?

Because he's a badass three headed dragon.

Is he the strongest?

Not by any means.

So yes fans, sad to say it but Godzilla's single most powerful opponent is in fact Destoroyah. He may not be the most popular, but he is hands down the single most powerful opponent Godzilla has ever faced in the entire franchise.

Had he been in Final Wars he'd likely have been the final boss instead of Monster X. The most likely reason they didn't use him was because of the fact that Destoroyah was created with the Oxygen Destroyer, and that device was never mentioned in the film nor even referenced. It's likely they ignored its continuity altogether becuase almost every other monster came back save for Biollante, Spacegodzilla, Mechagodzilla, and Destoroyah. Each one has unique and special circumstances which make it impossible to force feed into such a story, even one as spastic as Final Wars.

So arguments can be made, but the facts all remain. No other opponent has pushed Godzilla as far as Destoroyah did, and for those keeping track, I count Mechagodzilla II aiming for Godzilla's secondary brain as cheating on the human's part. Had they let Godzilla continue fighting and left that brain alone, I doubt it would have taken Rodan's energy to win the day when Godzilla defeated him with a single reverse charge during their first fight. If given enough time, Godzilla could have won the second fight the same way by sending that charge back to fry Mecha-G's circuits.