Hi All!
I have three "best bikes", not so much based on spec', but on what they have done for me and my enjoyment/accomplishment in cycling.
Chronologically...
First would be my 1977 Nishiki Olympic 'cos it was my first "real" bicycle, to my way of thinking. Compared to the Little Red Bike hand-me-down boyhood bike, the Schwinn Sting-Ray, and the Schwinn Varsity with its wicked-great amounts of toe overlap and magnetic attraction to guardrails, the Nishiki road like a dream. It was my Freedom Machine and I rode it regularly 8,000-12,000mi/13,000-19,000km each year as an under grad at Uni. Though it "only" had a plain-gauge high-carbon steel frame and steel rims when I got it, it also had gorgeous lugwork, great build quality and geometry, the sweetest ride, and was relatively lightweight at 28lb/12.7kg. I rode it 22,000mi/35,000km, then my father added another 13,000mi/21,000km of heavy loaded touring, and it never let either of us down. It is hanging in the garage now, each of us too sentimental to ever let it go. And why should we? With decades of upgrades, it still rides nicely and is a real pleasure to take out from time to time.
Second of my all-time "best bikes" is my blue rando bike, a 1983/84 Centurion ProTour 15. Comfortable carrying a maximum touring load of 56lb/26kg on logging roads, it also never leaves me feeling beat-up after a 400km day on pavement. Viceless handling, responsive, and relatively light at 33lb/15kg, it is the nicest all-'rounder I've ever owned. With well over 28,000mi/45,000km on the clock, it still feels new to me and brings a smile with every ride.
Third on the list chronologically is my most recent bike, the Thorn Nomad Mk2. The ideal expedition touring bike I'd been looking for to carry sometimes-enormous loads for extended stays in the back-of-beyond, it is now also a reasonable and fun if somewhat heavy all-'rounder, thanks to the addition of a Thudbuster seatpost to take the edge off bumps when riding unladen. I like the Rohloff particularly well because it almost perfectly duplicates the durable half-step-and-granny/5-cog gearing on the blue rando bike, one of my all-time favorites. The low-maintenance reliability is icing on the cake. It makes a workable road bike that allows the freedom to take any little dirt track I happen to pass and makes both enjoyable.
The tandem would have to figure in here somewhere as a "best bike" of sorts, but doesn't see the use the blue rando bike and Nomad do. Between those two, the whole spectrum of nearly all my riding is nicely covered and as a result, the other bikes just don't see as much use as they once did.
Best (bike),
Dan.