I normally don't go in for rankings, but in this case it is simply beyond dispute that Eddy Merckx is the greatest cyclist ever to throw a leg over a bike. He won each of the sport's three big grand tours: the Tour de France 5 times, the Tour of Italy 5 times, and the Tour of Spain once (the only time he entered it). His total of 11 grand tour victories has never been matched. Merckx also won 28 "classics" races, including every single "classics" event except Paris-Tours. Of those "classics" wins, 7 came at Milan-San Remo and 3 at Paris-Roubaix.
Merckx also dominated on the track. He set the hour track record and won three European track championships. A full tally of Merckx's accomplishments can be viewed here.
In 1971, Merckx won an astonishing 45 percent of the races he entered, including wins in the Tour de France, the World Championship, Milan-San Remo, Liege-Bastogne-Liege, Paris-Nice, and the Dauphine Libere. Other riders used to joke (not very ironically) that if Merckx showed up for a race, everyone else was racing for second place. Arguably, no sport has ever been so dominated by a single individual as Merckx dominated cycling.