Each country has its own beauty, but if you want to sightseeing, Vietnam and Laos will be the best places among such places. Cambodia is just great for temples such as Angkor Thom and Angkor Wat. Thailand offers fewer beautiful landscape than Vietnam and Laos, but its has the best service. For me, if you have a month for vacation, you should spend on 4 these countries, especially visiting Vietnam and Laos.