Argentinasaur was discovered about 13 years ago...it's a long neck and ,if ya couldn't tell by it's name, was found in Argentina.
However while the Argentinasaur is the heaviest it's certainly not the tallest or the longest. The tallest is Saroposeidon, which is listed in the Guinness Book of Records as officially the tallest of the long necked species.
The longest is the Bruhathkayosaurus at 144 ft/44 meters (compared to the Argentinosaur which is 30 meters) altho Wikipedia says it's the Supersaurus that's the longest (131 ft). But this website I found says it's the Bruhathkayosaurus:
http://www.gavinrymill.com/dinosaurs/giants/giants.html