Q9550. Running at 3.4 GHz slightly below stock voltage for the last 3.5 years without any complaints.
I should have kept my two Gigabyte X48-DS5 boards and my two Q9550 CPUs. I could have been sitting pretty for quite some time on those.
I still have my two GA-P35-DS3R v1.0 boards, which I upgraded from E2140 CPUs to Q9300 CPUs. Unfortunately, these boards don't have much headroom above 400MHz FSB, so my Q9300 CPUs are limited to 3.0Ghz.
I also purchased, between these two, two DFI X48 boards and Q6600 CPUs.
I sold one of the X48-DS5/Q9550 combos for a slight profit on Craigslist, and then I used the other one to build a gaming rig for my BIL with two HD6870 cards.
I could have been rocking 3.6 Q9550 chips for a while. Sigh.
I did also purchase an X6 1045T at MC in a combo with an ASRock 990FX Extreme4 board. That rig was also very nice, especially for DC, but the power consumption was pretty high. So that combo is for sale in my FS thread.