Two by two:
1. Biggest Sauropod: Argentinosaurus
Alain Beneteau
Although paleontologists claim to have found bigger dinosaurs, Argentinosaurus is the biggest sauropod whose size has been backed up by convincing evidence. This gigantic plant-muncher (named after Argentina, where its remains were found) measured about 120 feet from head to tail and may have weighed over 100 tons. Just one vertebra of Argentinosaurus is over four feet thick! (Other, less-well-attested contenders for the "biggest sauropod" title include Futalognkosaurus, Bruhathkayosaurus and Amphicoelias.) More about Argentinosaurus
2. Biggest Sauropod (tie): Sauroposeidon
H. Kyoht Luterman
Named after Poseidon, the Greek god of the ocean, Sauroposeidon was once thought to be the biggest dinosaur of all time, but paleontologists have since concluded that it was slightly lighter than Argentinosaurus, "only" about 50 or 60 tons. However, Sauroposeidon was almost certainly the tallest sauropod ever to roam the earth; its neck alone was almost 40 feet long! More about Sauroposeidon
3. Biggest Carnivore: Spinosaurus
Nobu Tamura
You probably thought the winner in this category would be T. Rex, but it's now believed that Spinosaurus (which had a huge, crocodile-like mouth and a sail of skin jutting up from its back) was slightly heavier, weighing in at 7 or 8 tons. It's possible that this dinosaur's famous sail evolved as a way of increasing its skin area, and hence allowing it to cool down faster--yet more evidence that Spinosaurus was the king of the meat-eaters. More about Spinosaurus
4. Biggest Pterosaur: Quetzalcoatlus
U.S. Government Services
Just as Sauroposeidon is named after the Greek god of the ocean, Quetzalcoatlus is named after the winged Aztec god Quetzalcoatl. This gigantic pterosaur had a wingspan of up to 45 feet, making it the largest creature ever to fly, modern eagles included. That is, if Quetzalcoatlus really did fly: new research hints that this giant pterosaur may have led a completely landbound existence. More about Quetzalcoatlus
5. Biggest Pliosaur: Liopleurodon
Adam Stuart Smith
With its long, thick, tooth-studded jaws, bulky body, and massive flippers, this pliosaur looked a bit like a cross between an orca and a shark. Paleontologists believe Liopleurodon attained lengths of 40 to 50 feet, and may have weighed 20 to 30 tons, about the dimensions of an adult sperm whale. If this doesn't sound impressive, keep in mind that the biggest great white sharks weigh about 3 tons, max. More about Liopleurodon
10 Biggest Dinosaurs:
The 10 Biggest Dinosaurs - The Largest Dinosaurs, Pterosaurs and Plesiosaurs