Scythe Mugen 2: I achieved 37c at 7% load with my 6 core AM3 by adding a second fan. (same model number as the stock one, both configured to run at 675 RPM)
The Mugen 2, as well any high end tower cooler, with the side panel fan mounted it won't fit (that case should have at most 13.8cm, less than 5.5 inches, clearance).
Apart the OP don't need such a fan, at anyway the best heatsink which clears this requirement is the very expensive Noctua NH-C14.
Depending of the CPU, it may be an overkill or not.
Second best, according to me, would be the slightly less expensive Thermalright AXP140 paired with something like a Scythe Kama Flow PWM fan (or, maybe, with a Thermalright TY-140 one).
Eventually, if the OP should remove that pointless, almost counter-productive side fan (it should be an exhaust), then the Mugen 2 would be a very well balanced strike (usually substantially less expensive than the above mentioned options).