China
Reasons .. dangerous or too hot or culturally not attractive to me.
I have just come back from a 3 week holiday in China.
As a tourist I felt very safe. We were sometimes in really crowded places - tourist sites, subways, people out having a good time on the streets in the evening - but there was never any sign of trouble or rowdy behaviour and no fear of getting your pocket picked or your bag snatched. I would be much more on my guard in Paris or Barcelona or any major European city.
Temperature depends a bit on time of year and where you are, it is an enormous country. We started in Beijing and gradually worked out way South ending in Hong Kong. At this time of year the temperature in the North was similar to UK. As we moved South it got a bit hotter, low 30s, but never unbearable. I think some areas can get in to the 40s in summer. Pick your time of year carefully. We spent one night in a monastery in the mountains and had to sleep fully dressed to keep warm!
Culture-wise a very, very varied country. Interesting cities, breathtaking landscapes.