When it comes to topics like this, I usually try to avoid mentioning recent things because not enough time has passed to fully judge them yet, but I legitimately think my current 5800X3D might end up being it. In an era where chips aren't advancing as fast as they did in their prime (late 1990s and early 2000s), it's amazing how much of an uplift this was over the 3700X I had before it, which wasn't even that old at the time and itself was one heck of a value when I got it (two-third the cost of the 10700K while generally being 90%+ of the performance, and that's before mentioning the savings of the included cooler). The 5800X3D was the first of its kind, and it came as the capstone to the AM4 platform. The AM4 platform as a whole dethroned LGA775 for its legendary spot, and I agree with somewhat said earlier; I think that once time has passed and we can judge them more fully, a lot of the Zen 2 and especially Zen 3 CPUs will be easy mentions for this.
My older Core i5 2500K has to get a mention too simply due to how long it lasted me (very late 2011 until mid 2020), although that could also be a bad thing because it happened precisely because of how slow CPUs advanced in the mid and late 2010s. Before Intel launched the 12th generation, my opinion was that Intel had no noteworthy release after Sandy Bridge.
Okay, so... if the first is too recent to count, and if the second counts for longevity but is bittersweet because of why, then a third mention would basically be any of the old AMD Athlons from the very late 1990s and early to mid 2000s (Athlon, Athlon XP, and Athlon 64 X2), but perhaps the Athlon XP in particular. I was getting into PCs at the time, and that was sort of the CPU everyone seemed to have/want/recommend, and so of course I also wanted one. I never got one, but I did end up getting an Athlon 64 X2 when a friend of the family upgraded and passed it on to me, but it was unfortunately short lived. Those were times of fast growth and close competition.
