i have a few, like going from a Pentium 120 to a Cryix MII 250MHz, though the Cyrix was crap, the clock difference was enough to speed things up drastically. From a 486 DX2/66 to that Pentium 120 was a big one as well. One of the biggest performance leaps for me was going from Slot 1, Socket 7 to Slot A and Socket A and Socket 370 era cpus, this in my opinion is where the platforms really started to get juicy. Leaving Socket 7 K6 cpus to a Slot A 950 was tremendous and i literally saw and felt the difference, maybe because hard drives were starting to get decently fast as well and memory was'nt that expensive anymore.

In recent times it has to be from Socket A to 939, 939 feels snappier and also shows a decent difference in gaming etc.