It was Avatar for the longest time. Now I just show off anything with Atmos.
http://thedigitaltheater.com/index.php/dolby-trailers/
You can stream each of these on VUDU through PS4 as well, just create a US playstation account and use a DNS proxy.
As far as movies... Many audiophiles swear by the Dolby Atmos mix of Gravity on Bluray - Which wasn't included in the original release, look for the 'Diamond Luxe Edition. I ordered it a while back and have re-watched several times, just an amazing sound mix.