I agree that if you want big open skies and clear views of mountains then Annapurna / Mustang area is great. Worth combining it with some Kathmandu valley rides to see a different side of Nepal. The countryside in the 2 places is completely different. I have biked with Kumar at Himalbiking – but I think you may be after more hardcore biking than he would normally do. But maybe have a look at the photos? http://www.himalbiking.com/gallery/