I agree with the atomic breath.
If you were to peg some of the details that have made Godzilla such a long-lasting icon, the atomic breath would be right up near the top. And actually I think the glowing dorsal fin may even rank slightly higher... I mean the whole fire-breathing dragon thing has been around a long time, but whoever said at that early production meeting for the first movie "Hey, when Godzilla uses his atomic breath, let's have his dorsal fin glow at the same time!"
That was just shear brilliance.
The other thing that makes Godzilla... well Godzilla?
The roar.
And that roar was almost lost to history because in the second Godzilla flick "Gigantis-The Fire Monster" the famous Godzilla roar is not used. Instead you have two creatures that sound the same... Ugh.