What country had the largest empire and the greatest naval fleet?
The largest Empire ever forged was the British Empire spanning 33.67 million km² (13 million square miles), about 1/4 of the Earth's total land area, and encompassing 1/4 of the Earth's population.
The largest Navy in history is most likey the United States Navy as of 1945 with over 1,600 warships. The most powerful Navy in history is modern United States Navy.
The answer depends greatly on definitions. By many accounts the Allied invasion fleet assembled for D-Day was history's largest, including roughly 5,000 vessels, and more than 100,000 people. If we define "largest" to mean greatest firepower, then the largest fleet might be that assembled for the attack on Iraq in 1990, which involved many nuclear aircraft carriers and a variety of heavy and light surface vessels.
The United States has the largest airline fleet in the world. The US is home to Delta Air Lines, it is the world's largest airline in passenger miles transported and passenger fleet. The US is also home to Southwest Airlines, Delta Airlines, United Airlines and Northwest Airlines. All of these airlines are in the top 10 by annual passengers carried.