For me, my Fairphone 3. I've upgraded the camera and changed the charging port after it got water damage, the phone is still going strong when any other one would have had to be replaced.
I have every intention to keep using it for many years more, before I struggled to keep a phone for more than a couple of years. :)
The company appears to be doing quite well, so for now I have no doubt they will continue to offer spare parts for a good while after the release of a device. Perhaps more importantly, the modular design makes it easy to buy parts from the community: By the time the phone is old enough that official parts are not available, finding old parts from other Fairphone users shouldn't be too challenging. The Fairphone forums still see quite a bit of trade of parts for the Fairphone 2, despite new parts having been out of production for a while. Newer models have also sold in much higher numbers, so it is reasonable to expect more parts to be on the market once they get old enough for users to start upgrading.
Batteries are a challenge as they will eventually get old, but the fact that batteries for the Fairphone 2 (launched 2015) were sold until April this year is uplifting. :)