If you are staying for just one full day in Iguazú falls, I would go for the Belmond in Brazil or the Meliá on the Argentine side. Never tried the Belmond and I was in the Meliá when it was still a Sheraton (I have heard it is a bit more run down now), but in both cases the location is unbeateable -- you can actually walk from your room to the falls. I did the Itaipú tour once as I was travelling with a then-girlfriend who was an engineer. If you are an engineer yourself, go ahead and do it, otherwise I would skip it, especially since you only have one day. Also I agree with [MENTION=538456]RafKa[/MENTION] -- forget about Paraguay; while Asunción has grown on me, Ciudad del Este is dystopian.

Agree with everybody that the Best Western seems to be in a very good location. Barra is of no interest in my experience. If you wan to splurge, apparently the Fasano is the new place to be or so tell me my chic friends, but I'm too stingy to even consider paying +400 USD/night anywhere in the world.