If you had only one lens on your OMD or other MF3 rig for walking around which lens would it be?

I get a lot of use out of my Canon FD 28mm 2.8. It works well as a close focus lens and as a landscape lens. At 28mm the focal length is pretty close to what's referred to as a "normal" focal length. As a prime lens it has much better optics than an equivalent zoom. I wouldn't mind if it was a little faster, but f/2.8 is faster than most zooms already. I don't mind zooming with my feet since zoom lenses are used most often at their extremes anyways. In other words, you put a zoom on your camera and end up zooming with your feet anyway. You pay for the middle ranges with more money and poorer optics but you don't use the middle ranges all that often.

YMMV.