There are only four riders who led the general classification of Giro d’Italia thtough the entire race. Although the number of days they spent in leadeship differs due to the different stucture of the race though the years.
Costante Girardengo 1919 (10 stages)
During the first edition of Giro d’Italia after World War I Girardengo dominated the race on an incredible level. He won 7 of the 10 stages.
Alfredo Binda 1927 (15 stages)
Binda won the general classification two years earlier and was one of the main favourites in 1926. Unfortunately, he crashed on the very first stage and gave up the hope to win the race. So he rather helped his teammate Giovanni Brunero to earn his (then) record-breaking third overall victory. Nevertheless, Binda managed to win 6 stages and finish 2nd behind Bruneiro.
In 1927, Binda demostrated already during the first 3 stages who is the undisputable ruler of the peloton. He won 12 of the 15 stages.
Eddy Merckx 1973 ( 20 stages)
The Giro d’Italia in 1973 has a special place in road cycling history.This was the first time a cyclist managed to win both Vuelta a Espana (a spring race untill the 1990s) and Giro d’Italia in the same season. Because Merckx was won both Giro and Tour in 1972, it resulted an astonishing four-in-a row chain of grand tour victories.
Gianni Bugno 1990 (20 stages)
Bugno won the opening individual time trial, then a plain stage and the third (and ladt) individual time trial again.