Gonna give this post a bump.
I also can't easily pick a no 1 bike. I love the ride of all my 531 framed bikes. I love the ride of my Columbus steel bikes as well. But I suppose if I pick the bikes I most get a kick out of, I could narrow it down to two:
Mercian Olympic I believe. Bike as refurbished with new cables and housing. It now has tan wall tires too.
1973 or so Peugeot AO-8, vastly modified, running 650b x 42 mm tires.
The Mercian not only has the wonderful ride of an English 531 frame, it has by far the most patina of all my bikes. I tried to clean it up but the paint itself seems stained. So I built it up with other beat up and scarred parts. It's just a hoot to ride.
The Peugeot . . . well, what can I say? It took me by surprise when I first tried it out with decent aluminum wheels, so I kept modifying and improving it bit by bit. It is definitely the most comfortable ride I have with its long wheelbase and fat tires, but it also perfectly reflects what I love about old French bikes.