The three most common types of filling for a celebration cake is an all-butter sponge with jam and cream filling (essentially a Victoria Sponge), chocolate cake, or fruit cake (like a rich tea loaf or Christmas cake).
I would imagine chocolate is the most popular but recently I've been to a few parties where the main cake was a different flavour and enjoyed the change. The flavours I salivated over were carrot cake, red velvet and a ridiculously moist banana cake.
Helen from Helen's Designer Cakes makes cakes in the most delicious combinations - White chocolate mud, lemon lime white chocolate mud, caramel mud cake as well as carrot, vanilla butter, coffee walnut, orange coconut and many others. She is based in Queensland - her website is www.helensdesignercakes.com.au