A good way to think about this is ... What format do I use the most?

 Then use that as the primary one that you supply and maybe offer two additional in common exchange formats.

For example a person who used Cinama4D uploads thier stuff in C4D and then also offers it in OBJ and 3DS. That would be thoughtful posting for the site users. Also people have to stay away from the propriatary formats like MAX. Since 3DMax users are the only ones that can use it, it limits who can download it. OBJ, 3DS, MA, LWS come to mind as probably the most common used in 3D in the hobby arena and those are the people we are cartering too. Be kind to them, offer them a quality product and you'll get your downloads.