Just to be clear… you are asking what the offline dailies proxy file format should be, not what is being captured in camera, correct?
Camera will most likely be ProRes 444. Maybe ARRIRAW (if so, plan for a 5x increase in storage, backup and transfer times) or possibly DNxHD MXF if that camera has the additional $5K option. Even so, if choosing between ProRes and DNxHD for camera master I would go ProRes as you will get an additional 20 minutes per card.
When it comes to offline dailies, I would recommend that whatever you are shooting camera master wise be transcoded to an MXF wrapped DNxHD as there is no real advantage toProres in the offline process and DNxHD is optimized for Media Composer where ProRes can me Avid managed, the codec itself is most likely not as optimized as DNxHD inside Media Composer.
Michael