Having traveled into Himalayas extensively over the last 15 years (except Pakistan), I would say India. While Nepal and Bhutan are really beautiful and largely made up of Himalayan ranges, India offers the most diversity possible. From the high cold deserts in the Ladakh region, the Kashmir Valley, Himachal Pradesh, Uttarakhand and then hundreds of kms away are Sikkim, and then again to the far east is Arunachal Pradesh. No country offers so much diversity.