Welcome to FlyerTalk. As some others have said my answer can change by mood, minute and the last place I visited.

After all is done it is still the Amazon for me, with particular emphasis on the jungle hotels near Manaus where the Rio Negro and Rio Amazonas merge http://www.brasil-turismo.com/fotos/rio-amazonas.htm the link is to a NASA photo that clearly shows the Negro continuing with the Amazonas beside it, running independently for a long distance before finally merging. An unbelievable experience!