One day race: Parix Roubaix (for the attrition and strain, for the pave of course and the gasp before the roar of the crowd at the apotheosis of the velodrome finish - it's the Hell of the North, need there be said more), followed by Strade Bianche (incredible parcours and scenery and, if it rains, unbeatable drama - or at least equal to that of a rainy Paris Roubaix - the meteoric rise of which is just so, so justified and Italy deserves a so beautifully landscaped race) and the Ronde (incredible show and so beautifully representative of a region and it's people). Each of these events, for their particular beauty and striking character, are veritable archetypes of bike racing.
GT: Giro d'Italia (because I live in Italy, but am not from Italy, and it is almost always more interesting to watch than the Tour de France - although le Tour is le Tour as they say, even if I can no longer have the pleasure of reading Gianni Mura's (R.I.P.) brilliant articles covering le Grand Boucle in the midsummer heat of lazy July). Each region, from Sicily to Campania, from Lazio, Umbria, Toscana and the Marche, from Liguria, Piedmonte, Lombardia, Trentino-Alto Adige and Veneto offer a combination of incredible terrain with beautiful medieval towns to race through. Plus the tifosi along a Dolomite pass or at la vetta of the Stelvio or Zoncolan electrify the spectacle.
Other races: World Championships (incredible competition and dramatic build-up to the climactic final laps), Milano-San Remo (la Primavera lives up to its Botticellian-Vivaldian mythos, a mixture of peculiar beauty, melancholy and poetry, which along the Ligurian Riviera is no less of a spectacle than Roubaix, Strade and Flanders, while no cycling season can be said to have "officially begun" without it), Tirreno-Adriatico, Paris-Nice, Dauphine, Lieges, Lombardia