Hi Julia,
I voted for Nepal but it was a close run thing between Nepal and Mongolia for me! I'd love to visit Nepal to see the Himalayas, Phewa Lake and Chitwan NP and hopefully will get there over the next few years. I really want to see Mongolia as well and will be keeping an eye out for a review from Jonah when she comes back. However, I can't go on holiday during July or August so hopefully if the tour continues to be popular, JY might put on an earlier May/June or Sept departure in a couple of years. Providing Jonah reports back that the gers are comfortable and the food is decent! The scenery should be amazing there.
Out of that list, I've been to China, India, Vietnam, Cambodia, Borneo and Burma so I've seen quite a bit of SE Asia at this stage. My highlights have been the Great Wall and pandas (China), the Taj Mahal and seeing a tiger very close up in Ranthambore (India), lovely scenery at Halong Bay and near Hue (Vietnam), Angkor Wat temples (Cambodia), seeing orang-utans and pygmy elephants (Borneo) and the diversity of temples and landscapes and friendly people (well really, everything, in Burma). Although not on your list, I really loved Singapore for the wealth of interesting things to do, innovative buildings and the fabulous zoo (breakfast with the orang-utans!). Hopefully I will get to Thailand and Laos one day as well.....
All the best,
Bob