for me thailand and vietnam would be the best options for an overall experience which you describe, but i would pick one or the other in order to not "rush"...
thailand generally has better air service into it than does vn.. more flts and cheaper...
both can be reasonable for every day expenses. both have fantastic sights... thailand generally has a more developed infastructure..
personally i prefer thailand and find more depth to the experiences there.
while cambodia is developing, it is way behind the other 2. siem reap, the locaation of the angkor war ruins is the country's primary sight...