Canada
Cold. Shitty. A lot of mountains, forests and places where noone lives. Army trained in cold and shit, while US is not. Modern equipment. Supply through Arctic.
Warm. Shitty. A lot of mountains, rainforests and places where noone lives. Army is a fucking joke. Next to no equipment. No supply.
Sure, yeah, guerilla forces yes. The thing is that Canadians could hold their own in the rough terrain with their army at least until they get help from outside, while South Americans would be forced to use bunch of dudes with pointy sticks jumping out of trees to fight USA, while they would get no help due to USN murdering any tries to supply them (inb4 hue narco-subs hue).
South America combined is not "a bunch of dudes with pointy sticks jumping out of trees," and I'd love to see your citation for that. The Canadian army, numerically, including reservists, ranks somewhere around not South America as a whole, but maybe just Argentina alone, and numbers are going to have a powerful weight in this kind of conflict. Against the preponderance of force the US can apply in the early years of this hypothetical conflict, especially to its northern neighbor, any "stand-and-fight" mentality is going to result in that army of less than 50k soldiers and reservists melting away in the wind. Quality-wise, they're far superior than any South American army, but those millions of South American soldiers and guerillas aren't just armed with sticks, and a bullet from an M964 FAL, M4 (which happens to be intended for US standard issue as well), or even an AK-47 will kill someone just as dead as a bullet from a Canadian C7. The only way either Canada or all of the countries of South America are going to stay in the fight is through guerilla warfare, so don't denigrate it, and don't pretend one side's guerilla forces don't count while another side's do just because the latter's a First World country and the former aren't. The US is certainly not going to have an easier time conquering an entire continent half a world away compared to a single country right next door.
Also, how is supply getting over the Arctic in the amount required? Last I checked, it's kinda covered in ice. Cargo submarines were mentioned before by you, but these have never been a major part of any nation's naval arsenal due to their inefficiency. Icebreakers, you mention, but these are basically target practice; break the lead ships and the entire fleet is stranded to be picked off at leisure. By this general burst of logic, I could go "lol icebreakers" (by analogy to your "hue narco-subs") and say that supply can reach South America by circling Antarctica. And lo, suddenly South America is magically as well-supplied as Canada. Besides, in seriousness, there is much more coastline to cover for South America as a whole than there is for Canada, so that's not even such awful logic as one might think.