I absolutely love studying ancient art and architecture. I find it fascinating that almost the entire collective knowledge, beliefs and social conditions can be understood and interpreted by art and architecture alone. The classical and ancient styles of architecture are my favourites (aside from Art Deco), and I'm constantly building a library of stuff from these times.
I find studying traditional and vernacular design not only helps develop your potential to create but also to interpret the environment and contextual conditions differently. Where I live (South Africa) there is so much traditional art and influence on contemporary design in the creative disciplines, (architecture, fashion, interior design, industrial design), which it is quite exciting.