What one particular song best represents a particular country in your opinion?

But it is a beautiful song, I knew the Serrat version first. And thanks to your very fine translation I have finally found out that cambalache means bazaar. TIL.
Concerning @EinsteinsHund optimism, I will not join in. Ode to Joy, if only! I’ll be slightly more realistic: Eigekühlter Bommerlunder, by die Toten Hosen:

Hah, that was the unauthorized video shot that desecrated a church in Bavaria, which had to be consecrated anew after that abomination. It was a minor scandal back then, and as a Hosen fan I snickered a lot about it, back when they were still punks and before their later schlageresque songs got played on CDU election parties…

And was that Marianne Sägebrecht as the bride? I hadn’t seen that video in 40 years and never noticed it. It was before she became a rather famous actress, at least in underground cinema.

If it is not her it must have been her younger thinner sister, I hadn’t noticed either, but it seems obvious now you point it out.
She had a couple of international movie hits too in the '80s: Zuckerbaby, Out of Rosenheim and Rosalie goes shopping come to mind.

USA - Take Me Home, Country Roads

Mongolia: The Steppes by Steve Hackett. I think the song would be a perfect backing track for a series of slow aerial flyover shots of impossibly vast plains surrounded by distant snow-capped mountains.