I think right now my ultimate N=1 bike would be a Volagi Viaje. That's their steel model, so hopefully you can get a lot more years out of the frame than carbon Liscio; it's sleek like a road bike, and can be built to under 20lb, so no problem with road rides; but it also allows up to 42mm tires, has braze-ons for fenders and (rear) rack, and disc brakes, so it covers the commuter end, and could even do light touring or trails. And their 'longbow' seatstay design is supposed to be incredibly comfortable (so they claim).
So I think my answer would be, Volagi Viaje with a 700x42 wheelset for 'work', and a 700x25 wheelset for 'play'.