If you're talking about Notocolossus, the closest relative is probably Dreadnoughtus (unless it's an argyrosaur) or Drusilasaura. The paper says Dreadnoughtus came out as the closes taxon, but it didn't input Drusilasaura or most of the other lognkosaurs so as of now it's still debatable. But it does show a lot of similarities with Dreadnoughtus. I have more of an issue with how they had Tapuiasaurus coming out as the next closest taxon, even though it's a nemegtosaur... and it's only known from a handful of remains, the best preserved being the skull. Most of the other elements are pretty eroded and in my view not referrable to lognkosauria.
If you're talking about the Chubut Monster, I would guess the closest relative is either Futalognkosaurus or Traukutitan.