Even today architectural students study the work of Vitruvius, who was a Roman author, architect, civil engineer and military engineer during the 1st century BC, and is known for his multi-volume work entitled De architectura. His discussion of perfect proportion in architecture and the human body led to the famous Renaissance drawing by Leonardo da Vinci of Vitruvian Man.
Vitruvius is famous for asserting that a structure must exhibit the three qualities of "firmitatis, utilitatis, venustatis" – that is, stability, commodity, beauty. These are sometimes termed the Vitruvian virtues or the Vitruvian Triad. No building should be judged on the basis of just one of these qualities.
Nowadays "firmitatis" or structural stability, is left more to the work of structural engineers who architects rely on to ensure that a building can be safely constructed and will not fall down.
To many practicing architects "utilitatis" , sometimes, translated as "commodity", is the starting point for design. The architect is presented with a site and a brief, with the brief describing how the building is to be used and how it may have to adapt to future uses. We also need to design buildings that are truly sustainable and add, not detract from our environment. Quite a number of eye-catching prize-winning buildings by famous architects have later been criticised because they did not function as well as they should.
The final quality "venustatis" or "beauty, is the most difficult to assess, as it is to some extent, as the OP says, in the eye of the beholder. A building may be judged in relation to its context - geographical, cultural, historical. There are some aesthetic guidelines for design, but the definition of beauty can never be exact. A truly beautiful building will also be structurally sound and appropriate for its use. Visual appearance, both interior and exterior, may be delightful, but building cannot be truly beautiful unless all three qualities - of firmitatis, utilitatis, venustatis, are integrated in total harmony.
The RIAS is, I think looking for the most beautiful building in its entirety, not just for the one with the most aesthetic appeal.