Any itinerary that includes REP is a winner! Seriously, I can not overstate how much I recommend Angkor Wat. It was perhaps the most lovely experience from my time in SEA.
Although the OP has decided on China for this trip, we do agree on this. I've been multiple times but particularly like Siem Reap/Angkor in the rainy season--lush, jungly-atmospheric, relatively few tourists, lower prices.