I've been lucky enough to do the western US (Cali, Mont. Wyoming, Idaho, Colorado, etc) and Canada and Alaska (lived there...) And lucky to have done most of the Pacific Crest Trail and a chunk of the Appalachian trail. And they were all perfect experiences.
So for me, it would be Africa, more Africa and a bit more, topped off by Pakistan, Himalayan India, Nepal and Tibet. Yep, those are for me -- although as a Yank I reckon I'll have to lay off of the back country of Pakistan and Afghanistan for a while...
(New Zealand would be grand too...)
Now about Siberia...