To those who haven't visited, the 9/11 memorial in NYC is very powerful and well done. I don't think there's a dry eye around by the time you get to the end.
There’s a solemnity to it that I’ve rarely experienced elsewhere. There’s even something sorrowful about the acoustics of the space, than I wasn’t able to fully grasp rationally.
I haven't been there, but the holocaust monument in Berlin (The Memorial to the Murdered Jews of Europe) had a similar effect on me.