Hard to narrow down to just one for me, but in terms of sheer lifespan, I have to say "SunTour Cyclone MkII derailleurs"
If it's on lifespan, my Woodrup frameset (OK, perhaps not strictly speaking a component).
This was the first new bike frame I bought myself, in 1977. Still going after 76,000 kms, though I don't think anything apart from the frame and forks is original. And only occasionally used now, as I now have a relatively lightweight Rohloff bike that I use for the same role of lightly-loaded day rides.
Apart from the frameset, my longest lifespan components, which are still serving on my old 650B tyre utility bike, all after decades of use and from 50,000 to 65,000 kms so far are:
- a Brooks B66 Champion saddle,
- 1980's Deore XT 4 finger brake levers,
- one of my Sturmey-Archer S5/2 hubs
- a high quality steel handlebar (Tange).
A pair of Mikashima quill pedals with steel cages come close, I got these second hand over 40 years ago and they are still going after at least 47,000 kms, currently fitted to my "new" utility bike. Judging by the condition of the finish, they had already had significant use before I got them.
A couple of other components reached the 50,000 km mark, a GB Randonneur alloy handlebar that I pre-emptively scrapped a few years ago after about 3 decades of use (concern about possible fatigue failure of alloy), and a set of 170 mm Stronglight 49D cranks that I retired when I solved knee problems by changing over to using 150 mm cranks.
As far as influence on my cycling is concerned, I reckon the three most important were :
- wide range derailleur gearing. Not a single component, but a combination of TA chainset with a huge difference between small 28T and large 48T chainrings, freewheel (14 to 28T, which was wide range for the time) and wide range front and rear derailleurs (Shimano Crane, this was just before SunTour was widely available in the UK). This made cycle-camping in hilly areas possible without having to get off and push.
- Brooks saddles. They don't work for everyone, but for me a Brooks B17 (or B66 Champion) made it possible to spend whole days riding for several days on end.
- Sturmey-Archer S5/2 hubs. I discovered these about 40 years ago and still have 5, of which 3 are in service and the other two kept as spares. They showed me that hub gears could be a reliable and low-maintenance alternative for utility riding and even some touring, as long as it wasn't in mountainous areas, and led to me changing over almost exclusively to hub gears later on when Shimano 8 and Rohloff 14 became available.
Never managed huge distances with rear derailleurs, on the kinds of roads and tracks I used there was generally a piece of wire or other debris lurking in wait to terminate these before they completely wore out. And the Shimano Deore RD-M592-SGS I used on my last big tour with a deralleur bike is fairly worn after less than 5000 km, perhaps because of several hundred kms use on wet sandy tracks.