Vienna, if you go either for a ''concert of nations'' vibe or if you want your UN between NATO and Commintern
Geneva, for a surviving League of Nations (bonus point for being in a neutral country)
Paris, in the palace of Versaille for fanciness
"Lots of rubble, divided among four powers, home of Adolf Hitler as a young man--let's make it Vienna!" said nobody at all.
Look, it was just not going to be in Europe. Only a few European nations wanted it there. The US, the Soviets, the Latin Americans, China, Australia, all agreed it should be in the Western Hemisphere. They wanted to be reminded as little as possible of the failures of the League--"make a fresh start." And the USSR, which had broken off relations with Switzerland after the Revolution and had been expelled from the League over the invasion of Finland in 1939, was especially vehemently opposed to Geneva.
https://books.google.com/books?id=BDAVCgAAQBAJ&pg=PA85