This is a two part question:

1) What is the most common/ easiest to use 3D model file format to use within a game? The end product will most likely be programmed in a C family language. (I understand this is a question of opinion, I just want to hear opinions.)

2) I need to make a massive, (EXTREAMELY MASSIVE), 3D model using a lot of yet-to-be-programmed algorithms that will allow me to calculate the billions of verticies the model will need. However, I have no idea how to store this information in a standard format. Does anyone know how to save information into a file format, given all of the verticies (x,y,z), so that I can use it later, in part or in whole, within my program. I am expecting the model to end of being hundreds of Gigs in size, but am unsure since I don't know how to store the model yet.

P.S. I am looking primarily at the DirectX 3D model format .x vs. the .max and .3ds formats. .x isn't used as much but is geared more for graphics use and the .3ds and .max formats are just everywhere but are hard to finddetails for the binary organization of the file types.