Even if you don't consider the prehistory of the sport... the most successful Italian rider was the Campionissimo, Fausto Coppi.
Then there is Bartali.
And then perhaps we can discuss Gimondi, Nibali, Pantani, Moser, etc.
We were lucky to see the strongest Italian rider of the post-war period after Gimondi: Vincenzo Nibali.
A really great one of this sport.
He was never the strongest on the climbs, not even the strongest in time trials, not fast, very good downhill but perhaps not the best.
But he was at a very high level in all these fields (except perhaps sprinting ) and thanks to his great intelligence, strength of will and presence in the peloton all year round at almost all the important races, he managed to put together a trophy gallery (the one he showed to Roglic) that, with the exception of the great Felice Gimondi, has no equal among post-war Italian riders, but I say more: has few equal in all the history of this sport (not only Italian riders !!)
To understand how Nibali is a big of this sport:
Only 7 riders in history won all three GTs: Nibali is among them.
Only 5 riders in history won all three GTs and a monument: Nibali is among them.
Only 4 riders in history won all three GTs and more than one monument: Nibali is among them (the others are Merckx, Gimondi and Hinault).
ps: Moser was strongest in classics, but he won only 1 GT (Giro) and only because Giro organizer designed a Giro with few mountains and hundred of TT kms for him.
It's difficult to make comparison with the period until 50s, that was a different world. Coppi, Girardengo etc. are really cycling monuments, but maybe of a different sport