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

okay, so you apparently decided to bring the D800 on your trip. good.

is the E-M5 for quick non-photo-centric use then?  dinners, etc?  your previous question was whether to leave the D800 and only bring the E-M5.  granted, this was asked in Nikon FX so the opinions may be skewed... but I think just bringing the D800 is a good choice.  especially if you are throwing in a 50mm prime.  the E-M5 isn't exactly pocketable and the focal length you have is already similar to the 1.4/50 on the D800.

but to answer your initial question, the one lens I almost always go for when walking around is the panasonic 7-14.  extremely small setup compared to practically anything else with this focal range, yet provides very high quality photos.  the range covers my usual needs very well.  only downside is it is quite expensive.  perhaps getting one of the cheap panasonic 14mm off ebay would be a cost-effective trade-off.  or you could go for the olympus m.zuiko 9-18 for something close to the 7-14's versatility, without its price.