Its very hard to quantify which dinosaur was the biggest as all we know has been learned from studying fossils. These fossils are more often than not incomplete and experts can't agree on exactly how the fossils would fit together to form the complete dinosaur. Fossils also only contain a record of the dinosaurs skeleton and don't convey the soft tissues and organs that would have made up the dinosaurs mass.
However, several suggestions have been made as to which the biggest dinosaur was, these include the Dreadnoughtus and the Argentinosaurus, which were some of the largest Titanosaurs. the Titanosaur was a subgroup of the sauropod family of dinosaurs