The Collapse destroyed so much of earth, seeking out anything of value. One might assume that the only relatively safe places to flee to would have been the places that were vile and repulsive enough for the Darkness to ignore. Probably Texas or Florida.

Based on the lore entries and in-game dialogue I've found, the Last City is most likely somewhere in South America. According to the Colony's lore tab, there are terciopelos outside the northern wall. Terciopelos are a type of pit viper native to Central America and South America. Assuming they weren't brought overseas to another continent, this would restrict the Last City's possible location to those regions. Luckily, with the other pieces of evidence I have found, there will be no need for such assumptions. Bear with me though because I'm gonna be pretty thorough with this. When retrieving some abandoned supplies in the EDZ during the Calling Them Home adventure from the Red War, Hawthorne mentioned how she "dropped everything and flew across the ocean to try and help" when the City was attacked. This rules out Europe, Asia, and Africa as possible locations for the Last City, since there isn't an ocean between Europe and Asia, Africa, or other parts of Europe. Later, during the Lost Cryptarch quest from year 2's Festival of the Lost, Amanda Holliday refers to the EDZ as being "across the pond" from the Last City, which further supports the idea that the City can't be in Europe, Asia, or Africa. On top of all of that, the lore entry titled Protector of Ghosts from the Ghost Stories lore book mentioned that one of the Ghosts Micah-10 had been escorting through Russia had resurrected Andal Brask, who then traveled across the Pacific Ocean in order to get to the Last City. The lore tab of the The Tall Tale exotic ship depicts his arrival. Again, this is just further evidence ruling out Europe, Asia, and Africa. Now, this leaves Australia, Antarctica, and the Americas as possible locations. Antarctica is ruled out by the Chromatic Fire lore tab, which establishes Antarctica as being believed to be a myth by Guardians. Now it's just Australia and the Americas. Back in Forsaken, if you talked to Amanda after Cayde's death, she'll have some lines of dialogue responding to his death. In the very beginning of this, she mentioned how her mom died crossing the Panama ravine when her family was traveling to the Last City. If they had a means to get to the City by boat or plane, then I doubt they'd be traveling across Panama to begin with. This effectively restricts the Last City's location to just the Americas. Given Amanda's accent, her family is likely from the US in North America. So crossing Panama would land them in South America, which aligns with the presence of terciopelos around the City's walls.

I always assumed that the City was within walking distance of the EDZ with the Red War campaign. An IGN post states the Last City is near Twilight Gap. Going off movement of the Sun it is most likely in Nepal, some even say below Mt. Everest which is in the background. Amanda seems to be evidence that it is in South America. Now if she was in say the US (possibly) than maybe she took a boat from somewhere in Florida and sailed across the Panama Ravine to jump 'The Pond' of the Pacific to get to someplace like Nepal in Asia, assuming she was from America and not from the EDZ. If you do assume Amanda is from the EDZ than it would make sense for it to be in South America with 'The Pond' being the Atlantic rather than the Pacific. Ever since D1 I always assumed the City was somewhere in Old Russia near the Cosmodrome, so for me its in Russia somewhere in the Ural Mountains.

I remember this being brought up before and someone said it's located in South America, somewhere around Argentina or Peru. I remember Colony's lore tab being a big help Edit #1: Nevermind about Colony's lore, it's no help at all. Edit #2: Nevermind again, Colony does help. Terciopelos are native to Central and South America, and it mentions one near the Northern Wall.

It’s through the stargate 😂

Given its implied proximity to the Cosmodrome, I would think its in the Ural Mountains, but until we get more in-game info its doubtful at best. I like the South America theories, but at the same time its hard to say without more exact lore on the matter. I will have to double check the Red War Campaign because I think I remember Hawthorne saying we needed to fly over a sea or ocean to reach the EDZ and The Farm, which if true, is a point in favor of South America.

If I remember correctly Amanda walked to the last city and I believe some place in the Americas was mentioned, (I can’t seem to find the lore). However as earlier stated the city seems to be close to the cosmodrome. [spoiler]nibbles crayon[/spoiler]

Someone did science and concluded it’s in South America based on geography and sun position. The geography I can accept but unless the Bungie devs are way more clever than I give them credit for, they just made skyboxes that look good from the balcony of the Tower. I’m hesitant to believe they intentionally put the sun where it would actually be in a specific geographical location.

Ohio

Lol I was always under the impression it was in Europe, Switzerland in particular, cause EDZ is in Switzerland. Assuming Last City must be nearby.

Definitely not in Australia.

I'm not sure but I think ember reading that the D1 tower was in Nepal? Not sure about D2

Detroit...