Top of my list it has to be Strade Bianche. It has drama, early season unpredictability, a spectacular finish set in medieval town of Siena. Pave', dust, gravel, stinging little climbs followed by dangerous descents. The winner is usually the type of rider I admire the most: Gilbert, Cancellara, Moser.
Giro di Lombardia is another race I love watching. Its hilly course and stunning views contribute to the mystique of this end-of-season monument. The Ghisallo climb is iconic in this course often mired by foul weather which adds to the epic efforts of past winners.
Brabantse Pijl is the opener for the Ardenne Classics. I believe its course is actually better than Amstel or Liege, offering a bit of all types of terrain. Even in the wet it's an exciting race to watch. Winding, cobbles, sharp climbs.