The best would be a QX9770 (through its free multipler), short afterwards the Q9650 then Q9550 then Q9450.
They will be able to go beyond 4 GHz relatively easy.
The older Q6600 is capable of good overclocking, too.
But even the Q9550 has around 433 MHz more and its more modern architecture will help getting higher clock spieed ant gives it slightly more performance per clock.
To beat the Performance of the 2.66 GHz Q9450 you will need around 2.8 GHz on the Q6600.