Neuvation F100, Surly Cross Check, Van Dessel Holeshot
All I can say is that the only off road riding I have done is on an MTB so I am going to start off with a 34, not that that measurement means anything as it is not standardized across brands, and then move down as I become comfortable on those tiny tires off road.
If you are only gonna have one tire for everything, I'd go 700x34 or 35. I run 700x38s or 40s for single track and urban assault rides. That being said, Kenda is hooking me with 700x32 Small Block 8s for racing this winter...they make 'em in 34c.
I don't notice a huge difference in rolling resistance or cornering from 35c on up to 42c. The fatter ones will ride better and flat less because of the increased volume.