id probably say godzilla. if you look at their sizes how their represented, godzilla should be bigger, though on paper ymir is. i dont think godzilla has ever found a physical object hes been unable to move or destroy, so his strength is impossible to determine, he has however been able to throw the 30 000 ton king ghidorah around for a while and not exhaust himself (literally picking him up and smashing him down and then throwing him).
his intelligence varys from movie to movie, but i think the most powerful would either be gmk godzilla or final wars godzilla, neither had intelligence on per with most humans, but they both had alot of cunning and battlefield awareness, successfully taking on multiple opponents at once. and his atomic breath has been powerful enough to obliterate another monster the same size as him. look how he handled american godzilla in final wars. on top of it he has a healing factor to rival the hulk, and survived having half his chest blown off only to return with no hint of damage.
that said i dont know alot about ymir, and he might be strong against most class 100s, even thor, but can he lift at least 30 000 tons? hes durable, but almost nothing hurts godzilla other than other giant atomic monsters, missles, lasers and the like do nothing to him. can ymir say the same? im not being sarcastic, i really have no idea. that is some of what godzilla can do, what are ymirs feats?
In Antartica it favours Ymir, you couldn't simply melt him because he would draw power from the freezing tundra surrounding him.
Can't simply "smash" him because he can regenerate from an icicle, his strength is incaluable way beyond class 100, 30,000 tons would be nothing to him, so he can do Godzilla some damage, not to mention freeze him on touch.
Ymir is 1000 feet tall, Godzilla is what, depending on incarnation up to 100 metres tall? which is about 328 feet tall, so Ymir is quite a bit bigger.
Good battle but Ymir takes this, he is smarter, heals as fast if not faster and is on a battlefield which suits him better.
This battle, in the enviroment its in favours Yimir,