Everyone is looking for it and nobody finds it. The problem is that all bikes are designed to do a few things well. Wnat to cover 60 miles in three hours? Carbon road bike. Want to be able to walk when you get there? Steel Audax bike (But it'll take you 5 hours). Want to ride trails? A mountain bike. Want to commute as well? Cyclocross bike (But it has no suspension so you can't "Downhill")
With this in mind...
I bought my current bike in the run up to the supported LEJOG ride a couple of years back and for this it was spot on. A lightweight steel road bike with 28mm puncture proof tyres with braze ons for rack and mudguards.
I tried touring on it the following year and found it a little flexi at the back with panniers and a fair old amount of heel strike. This year I fitted smaller tyres and mudguards as I expect to do the Dun Run and I have masses of toe-overlap
For my current needs (in my head) I need this bike here. Although what would be useful on a daily basis is probably my old brompton.
Which brings me to Rule 12. The correct number of bikes to own is N+1 where N is the number of bikes currently owned.