If you are going by regions, then it depends on what general recipes have been, and are used in those regions, as well as what food stuffs are available for such dessert items.
If you are in the Northern Americas, then pretty much everything is available. In the USA, probably the most common filling is pudding or custard, the next most popular would be chocolates and then fruit fillings.
The British are fond of their custards and fruit fillings as well, if I am to believe what I see on some of the British baking shows.
Anything can be used as a filling, but like Norcalbaker said, it really depends on preferential tastes. Some areas might have a preference for fruit, other areas might have a preference for cream fillings, while other areas may like compote, jams, or sliced fruit.
Three popular "store bought" cakes here in the USA that are popular across the country are Oreo Cake, and Snickers or Reeses Cake. Oreo Cake is just chocolate cake with plain vanilla frosting with Oreos crushed up in it.....used for filling and frosting. Reeses or Snickers Cake is chocolate cake with peanut butter cream filling for the Reeses and caramel cream filling for the Snickers, and both usually have a peanut butter frosting.......although there are many variances to these cakes.
From what I understand about British cake, their main preference is strawberry or raspberry jam and a cream filling layer between layers of white cake. I think this is called a Victoria Sponge.
Mexico's favorite cake is the Tres Leche's, which is a cinnamon white cake soaked in at least three different types of milk and/or sweet creams, topped with whipped cream and fruit.
Japan has Daifuku and Dorayaki. Daifuku is like a little cake ball with usually a cream or fruit filling inside, although anything really can be used for the filling. Dorayaki is like a macaron, but basically two tiny pancakes with a filling between them.
The "theme" for most of the "civilized" countries it seems is fruit. I'm not sure about other countries most popular cake like desserts, but for these countries, it seems to be that either a cream/custard filling or a fruit filling are the most widely used and accepted.
Hmmmm....I hope that helps with the answer you are looking for???