This question is one of the most frequently asked to anyone who works in the world of shipping, and unfortunately there is no easy answer. It really depends on what you mean by biggest.
THE LONGEST- Name: "Knock Nevis"; Type: Supertanker; Length 458 metres
THE WIDEST- Name: "USS Nimitz"; Type: Aircraft Carrier; Width (Beam): 77 metres
THE HIGHEST GROSS TONNAGE (GT)- Name: "Batillus"; Type: Supertanker; GT: 275,276 tons
THE MOST PEOPLE ONBOARD- Name: "Oasis of the Seas"; Type: Cruise Ship; Capacity: 6,296 people (passengers and crew)