In my own (humble) opinion, it would be the Shimano Dura-Ace 7700 25th anniversary; I got mine back in 2000-odd and it has been on my summer bike ever since. Cosmetically, the components look absolutely stunning, the polish hasn't faded, on a bright sunny day is a almost dazzling.
Mechanically, it works perfectly, as you would expect; it has covered something like 35,000-40,000 miles.
Durability wise, the brake blocks are still original and show good life left; I am not quite sure how this is possible as over the years I've been down some seriously steep descents and the brakes have always been excellent.
I have a few alterations from the Dura-Ace groupset; I use Rohloff chains, in my view the best chain ever made, the one on the bike was fitted last year, after the one that it replaced was showing a bit of stretch, after some 17,500 miles, which would be totally impossible with a modern setup with 10+ speeds. Also, I have a Thompson Masterpiece seatpost as the Dura-Ace seatpost failed. And also the bottom bracket is a known weak point, as it is designed to be as light as possible, so the bearings fail very quickly, so I have an Ultegra unit which has been faultless.
So with a few deviations, you have a groupset that works perfectly, looks beautiful, and lasts seemningly forever. If anyone can suggest something that can top that let me know.