I guess I'm going more for engineering elegance. Elegance on the inside, more than the aesthetic outside. So in no particular order:
Suntour Cyclone M2 rear derailleur. In GT or short-cage form, this thing still beats the pants off even modern carbon in terms of lightness. Nothing's there that doesn't need to be. Picture stolen from DisraeliGears.
Suntour endless band clamp for shifters and front derailleurs. Light and secure and easy to make. Mostly made of air, in fact.
Triple-butted spoke, 13-15-14. Why are all spokes not triple-butted? The threaded section is 14g, as big as it needs to be. The rolled threads bring the threaded section's maximum diameter to 13g, which is therefore the diameter of the hole in the hub flange. Why not fill this hole all the way up with a 13g elbow, so it's less likely to break? Then make the middle 15g, because anything thinner would be too stretchy. How much more will this actually weigh than a traditional 14-15-14 spoke? Why are they not all like this?
Sturmey Archer Dynohub. No more moving parts than a normal hub. But it generates basically-free electricity, by some small miracle! What they must have seemed like when they came out! The mind boggles. Many are still doing commuter duty today. I wonder if the modern Schmidt, Shimano, and Shutter Precision dynamo keepers-of-the-flame will be doing the same in half a century or more.
MAFAC and other stamped sheet metal cantilever. Right down to the pad holders, a tour-de-force of industrially manufactured minimalism.
Stronglight Delta. This headset is lighter, more durable, and better-sealed than its contemporaries, or indeed much of we have now. Plus, it stops shimmy. When they stopped making these, I became convinced that the free market was incapable of selecting for the best products or delivering them to consumers.
Double-eyeleted rims. You have two walls. Why not spread the load to them both? How many grams will this add, and how much strength? See the above about the triple butted spokes. It's a wheelbuilder's version of the platonic ideal, and sort of a free lunch.