Well, that changes, because each new (old) bike I rebuild is very enjoyable in its own way. My Suntour Vx equipped Grand Sprint (plain gauge Vitus 888 tubing) is very relaxed, very chilled ride without being sluggish. My 600EX SIS equipped daily commuting Moto (Reynolds 531 main tubes and forks) is tough as nails and can pack a punch when needed. So I'm guessing when I get round to rebuilding Columbus SL tubed C5 I have awaiting its turn to be made into vintage lightweight, I might change my mind. But I do have to say my last acquisition is absolutely fantastic. The frame is still somewhat unknown (probably Matsu****a made, possibly custom order Panasonic from 1982/83). It's Tange Champion 2 tubing with Suntour Pro dropouts and very well made. It came to me nearly complete, missing seatpost, saddle, inner tubes and tyres. It's equipped with full Suntour SuperbePro group dated 1981 (derailleurs, crankset) and 84 (hubs). Rims are Ambrosio 19 Extra Elite, I suspect originally the bike would have been equipped with Araya rims, but for whatever reason they were replaced at some point.
I had to replace BB cups with Sugino equivalent (Sugino made the BB for Suntour with Superbe branding, so it's essentially the same thing with a different engraving). I have replaced the original shifters with mid-80's ratcheting ones from the Sprint gruppo (the older, pure friction ones will be used in another rebuild). I've cleaned everything up, polished some bits that required it, regreased and put it back together. Still requires taking care of the paintwork at some point and the components do have some signs of chipped chrome etc. So not a picture perfect bicycle.
But the ride quality is absolutely fantastic. The whole thing at this point weighs 9kg or so. For whatever reason even riding up the measly hills around here with much higer gear at the back than I'm used to with my other bicycles feels better. It's very responsive and stable at the same time, takes corners at speed like nobody's business (but it's also good quality tyres). Shifting works great and is very precise. Sure, no indexing, but with this thing you absolutely forget that indexing exists and don't miss it at all. In fact, I think indexing might be a downgrade. Breaking is great and efficient, I'm very impressed. It's totally on par with modern, dual pivot calipers. And I'm still using the original brake pads. They did squal a bit (well, a lot), but after few minutes meeting with a piece of sandpaper the squaling is gone. There are some small things I'll have to do, like getting some nice tan-coloured brake hoods and probably a slight modification of the FD cage to reduce the need for trimming the FD shifter. I need to replace the seatpos with something more appropriate for the early 80's and the saddle with Italia Turbo or some Ariake. These are minor things though. 40 years after it was made, it will definitely be hitting the road as much as the weather allows. Honestly, modern carbon frame bikes might be still a bit lighter, but it would take a big wad of dosh for me to willingly swap for one.
Matshu****a / SuperbePro (?)