The Top 10 toughest bike rides list was taken by Lonely Planet. These are top 10.
Ho Chi Minh to Hanoi, Vietnam: The bike trip begins from lush plain of Mekong Delta, winding through stunning mountains around Da Lat, with hard climb through the Hai Van pass, then reaching Northern mountainous provinces. Besides absolutely abrupt routes, the searing tropical heat will make cyclists drop back into state of endless water refill.
Travelers indulging in Vietnam cycling usually challenge themselves to take a biking Ho Chi Minh trails – the Vietnamese famous historical road. Active Travel Asia (http://activetravel.asia) also recommend travelers exciting motorcycling tours (here) on this road.