If you’re based in Cambodia then your options are a bit limited for the countries you want to travel to unless you are willing to pay more for an indirect flight.
Bangkok Airways and SilkAir would top my list. Thai Airways and Vietnam Airlines wouldn’t cause me to lose much sleep either. Malaysia Airlines had two disasters in one year which totally messed up their record. The first one is yet to be explained, the second was simply wrong place at the wrong time as other airlines were flying the same route.
Which ever airline you choose is going to be safer than the long distance bus, especially the overnight ones.