My "one bike to rule them all" is a Surly Cross Check, but here's the trade-off:
It's pretty good at everything, but perfect for nothing.
It's a comfortable brevet bike, but it's on the heavy side and not designed for carrying a front bag so it can be a little wobbly above certain speeds depending on the load I'm carrying. For really long distances, the higher center of gravity can start to feel "tippy" when you're tired.
It's a comfortable century bike, but definitely overweight when it comes to climbing against the speedy people on carbon dream machines.
It's a good commuter, but the stays are short for carrying full panniers and there's no off-the-shelf option for a front low-rider rack.
It's a decent light-tourer, but again, see the pannier comment.
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"I feel like my world was classier before I found cyclocross."
- Mandi M.