Vitruvius’ Triad consists of firmitatis/stability, utilitatis/utility, and venustatis/beauty. I would say that most important is firmitatis, stability. This is because without stability of these ancient buildings, they may have been easily collapsible and we may not even know of their existence, should they not have survived. I think that utilitatis is least important, leaving venustatis to be of middle importance.
I place utility last because a structure may be beautiful and have no utility but would still be enjoyed at very least for its beauty. However, an unappealing building with no purpose becomes a waste of space and a potential eye sore.