Prime lens.

If it helps at all...

I have a 5D and some nice lenses but I prefer using my Panasonic G1 for everything except night time shooting when the G1 becomes unuseable as the EVF goes black and shines a torch in my eye but other than that I reach for the G1. I have a Panasonic 20mm f1.7 and a 14-42mm kit lens but I mostly use a Voigtlander 25mm f0.95 and Rokkor and Zuiko lenses.

You may know that MFT is x2 crop so a 25mm will give you the same FoV as a 50mm on FF. Once you've grasped that you'll possibly be using wider lenses you'll understand the DoF issue but if shallow DoF is your goal you can still get it by using wide aperture lenses and choosing your camera to subject distance, focal length and framing to get the result that you want. The crop factor has real positive effects too, for example when shooting in low light with my G1 I can get a more hand held friendly faster shutter speed and/or lower ISO than with my 5D and still get the DoF I want.