Look Marmotte Gran Fondo Alps
La Marmotte is regarded as one of Europe's toughest events, and is a key event on many cyclists' bucket list. The event totals 174km of tough riding, featuring 5000m of climbing over some of the toughest climbs in the Alps (Glandon, Galibier, Telegraph, and finishing at the summit of Alpe D'huez).
7,5o0 cyclists tackle the event every year and the atmosphere is immense - entries sell out FAST, so be sure to get in early to guarantee your spot.
Here's the course overview: