Not long ago, Pal Scott (the Sloth with Five Foot-Digits) posted his take on a "one bike to rule them all" thought exercise (that he got from some other blog, but I'll let you chase that link via his post, because, hey, isn't that whole "Choose Your Own Adventure" linky-thing part of what makes the Internets fun?)
I've grappled and grappled with it until my grappler is sore, but to no avail. So I decided to cast my gaze to my navel and interrogate (that's grad-school speak for "figger out") why I have such a problem coming up with the one ideal bike (other than my innate squirrel-like tendency to hoard). And I think it's because my biking fun comes from two often-contradictory sources, and those don't play nicely together in one bicycle.
My first fun is going fast. Granted, with the physiological and motivational gifts I've been given, "fast" is a relative term. But within those constraints, I like the feel of the wind in my bald spot. I like the way the pavement goes by when I'm turning a big gear. And I like the sound of (relatively) skinny tires on smooth pavement (even though those buzzkills at Bicycle Quarterly will tell you that the sound is a placebo effect unrelated to real speed).
My second fun is running stuff over. Granted, with two bum shoulders and an aftermarket titanium femur, I don't do that with the off-roader's enthusiasm I had in my youth, but it's still a hoot to bash into the occasional obstacle with nary a care. I've channeled that into a more practical concern (or, more accurately, justified it) by treating it as a commuter necessity -- when you use a bike to get somewhere, it should be able to take a beating without batting an eye. And there's always perverse pride/pleasure in riding over something that non-bikers think can't be ridden, even if that something is just a little snow.
It's easy to put those two purposes into two bikes -- my go-relatively-fast is my Raleigh road machine, while my run-stuff-over is my fat-tired Swift folder. Neither one pushes the slider out to the extreme end of the continuum (with fatbikes on one end and 13-pound carbon racers on the other) -- in a pinch, the Swift could stand in on a semi-fast ride, and the Raleigh could get dirt on it -- but they're definitely horses for courses. Putting those contradictory ideas into one bike is where the challenge lies -- slide too far toward the roughstuff end, and the go-fast fun disappears. Slide too far toward go-fast, and your banging-around reliability is gone. Cyclocross bikes come close, but even they have to sit on the same spectrum... you can get everything from a monstercross tank to a leg-shaver's superlight race dream.
(You may note that I haven't mentioned luggage capacity. I'm in a "carry it on me" phase these days, so it doesn't enter as a design consideration. Both of my bikes can take racks, though.)
I will take up the Sloth's cause for long-reach caliper brakes, though. A frame that uses the Tektro 73mm-reach model to its fullest would be as close to my ideal one-bike as possible. Such a frame would look like a racer from ten feet while accepting everything from 28mm road slicks to 35mm studded tires to 45mm "small-29er" tires. I know you could just do that with cantilevers (or, shudder, discs), but I have an irrational dislike of the cable stops that cantis require. Build it with eyelets for fenders and racks (since I might grow out of this luggage-on-me-phase), use light (but not stupid-light) tubes, slope the top tube (I know, retro-dork heresy), and put vertical drops on the back, and we'll talk. I'd even accept non-steel tubing (more retro-dork heresy). As far as I know, that's a custom request these days, so I'll stick with my two bikes for now.