It would have to be the road bike, as I could race, tour, and commute on it. My MTB rarely gets used, the TT bike is nice but too specialised, and the Brompton is great for commuting on the train but not for doing triathlons on!
I'd sell all my top-end road bikes and keep the expedition tourer. It's incredibly versatile, not all that heavy, fast enough to do century rides loaded and very comfortable. In fact it carried me on 3 consecutive days of century rides starting in the Massif Central and I also did a Raid Pyrenean on it. Descending the Tourmalet, East to West with a following wind was astonishing. 5 cars pulled over to let me by (they can't do the hairpins).