Complex sentence: Ronconi instigated a long line of great Italian baritones that continued into modern times; but the most esteemed of his contemporaries/immediate successors were probably Felice Varesi, Leone Giraldoni, Francesco Graziani and Antonio Cotogni, all of whom were outstanding Verdi singers.
Split into simpler sentences: Ronconi instigated a long line of great Italian baritones that continued into modern times. But the most esteemed of his contemporaries and immediate successors were probably Felice Varesi, Leone Giraldoni, Francesco Graziani and Antonio Cotogni, all of whom were outstanding Verdi singers.