For me my current CPU, the i7 920 has been my longest serving, however, I had an i7 970 whcih could easily do 4,5 Ghz with all 12 threads. Simply awesome folding power in there.
Favourite chip from yesteryear was the one that got me into overclocking - an AMD X2 4200. I can't remember how far I got it now, but that was the longest serving desktop chip that I'd had for years at that point... and has only been superceded by the 920 in my current rig.
Wish I'd never sold that 970....