Depends on Prince and Performance.
Forget the E6600. It's outdated in price and performance.
If you don't want to OC at all,
The E6000+ and E6400+ are not bad deals as they will provide good stock performance for not too much money.
If you are willing to OC, Then you have two common choices.
1) The E2160 - This should OC to 3.2-3.4 Ghz and match the 6400+ with lots of savings and reduced power consumption.
2) The E6750 - This will OC to about 3.6-3.8 Ghz and annihalate anything of the X2 chips. There will be a slight price premium for this.