The heaviest tasks I do on my PCs is realtime multi-channel audio processing and running VMs, so this may be boring.
2010: Core2 Duo Wolfdale, no idea which one exactly.
I built this one with 0 mechanical parts after getting fed up with keeping PC noises out of audio recordings.
It had a 400W fanless PSU, some ridiculous fanless copper heatsinks, SSD, cheapest video card I could find, and a Delta44 PCI sound card. I really wanted the Delta1010, but I was a cheapskate. Turning that thing on for the first time was almost creepy: pure silence other than the post beep, and pure silence loading the OS. I ran UbuntuStudio on that bad boy and it felt screaming fast until almost the very end. I think it was about 2016 that I noticed it start to struggle with Chrome and Youtube, but it was still a solid performer otherwise.
2020: Core i5-4200U
I've moved to using laptops as my main PC these days. Probably around 2016 or so, I got a used Lenovo T440 as my main rig: 1080p IPS LCD, replaced that awful touchpad for one with physical buttons, SSD, Behringer 4x4 USB audio interface, and still running UbuntuStudio. I can throw my entire rig in a guitar case and record wherever I want, which is a freedom I did not dare imagine in 2010. (I tried once by transplanting my desktop PC. 0/5 stars: would not recommend)
For years, I've been thinking about replacing my laptop, but to my dismay, I just can't justify it. Even now that I work in IT and I often run a couple VM's for work, the performance is still perfectly fine.