I would say it's between the Athlon II X2 245 and C2D E6600 as they are newer technology with a much smaller tech size of 45 nm and 60 nm vs 90nm on the Athlon X2 6000+. This should give more room for overclocking.
I would choose the C2D E6600 based on thie benchmark.
AMD Athlon II X2 245e vs Intel Pentium Dual-Core E6600
After doing a bit of reading, many say that the 245 and 6000+ max out around 3.6-3.7 Ghz.
The E6600 is the only one I could find hitting 4.0Ghz.