Obviously the world of cycling has changed over the decades, and what was possible 50 years ago simply isn't now due to increased competition and specialization. Still, the following seasons come to mind:
2011, Philippe Gilbert - the most dominating season in the UCI Pro Tour/UCI World Tour/UCI World Ranking era. Won 18 races, including Liège–Bastogne–Liège, Strade Bianche, Brabantse Pijl, Amstel Gold Race, La Flèche Wallonne, Clásica de San Sebastián, National TT, and National RR.
1987, Stephen Roche - Won the Tour, Giro, World Road Race, Tour de Romandie, Volta a la Comunitat Valenciana.
1972, Eddy Merckx - Take your pick, but the Giro/Tour double, 3 monuments, and the hour record is a season that may never be topped
Feel free to chime in with your picks!
2011, Philippe Gilbert - the most dominating season in the UCI Pro Tour/UCI World Tour/UCI World Ranking era. Won 18 races, including Liège–Bastogne–Liège, Strade Bianche, Brabantse Pijl, Amstel Gold Race, La Flèche Wallonne, Clásica de San Sebastián, National TT, and National RR.
1987, Stephen Roche - Won the Tour, Giro, World Road Race, Tour de Romandie, Volta a la Comunitat Valenciana.
1972, Eddy Merckx - Take your pick, but the Giro/Tour double, 3 monuments, and the hour record is a season that may never be topped
Feel free to chime in with your picks!