The intrinsic problem here is that a lot of guys can only talk about CPU's they've had and many don't have the same frame of reference of those that have been overclocking since the last century.

While the Q6600 is indeed a great CPU, with some guys getting to 3.8Ghz (58% o/c) I see a lot more guys getting around 3.5 Ghz or less. (46% o/c)
The Barton 2500+M would clock to around 2.4-2.7 (31%-47%) but was a cheap processor, something that couldn't be said of the Q6600.
I contend that greatest (overclocking) CPU of all time was the Celeron 300a. Not only was this chip comparatively very cheap at time but it would give you a
guaranteed 50% o/c to 450Mhz, the
only thing you needed to do was raise the bus from 66MHz to 100MHz. Even with the lower cache this would make it almost exactly the same speed in any application as the much more expensive PII 450Mhz.
Also bear in mind this was with the stock heatsink. (no need for water cooling or after market cooler) No need to increase the voltage. Around 30 people I knew at the time got one and all ran them with the 50% o/c.
The Q6600 would have to be able to go 3.6Ghz with the stock heatsink and no voltage increase. I think not.
Ladies & Gentlemen, I give you the Celeron 300a. Pound for pound the greatest CPU.