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

Far and away the most versatile lens I own.  Very good IQ also.

If I wanted something small and fast for low light situations, I would probably take the 14mm f2.5 to complement it. (If you prefer a narrower FOV, the 20mm would be another good option).

Actually, my normal kit when I go hiking is the 14-140mm and the 12mm f2.  Very light, and covers just about all the bases.  I have never been unhappy with either lens (or the 14mm f2.5 either, for that matter).  Around town I will generally use the 14mm alone as it is a great focal length for city spaces both indoors and out, and it is tiny.  However, if you are going on vacation and are going to want to do both close and far views, the 14-140mm is the way to go.

Of course, I have a Panasonic body, so you might want to consider the Oly 14-150 instead....however, I think the performance of the 14-140mm is better (tried them both, find the 14-140mm optically superior in several ways...sharpness, microcontrast, CA).

YMMV, of course....

-J