Assuming stock speeds ... not taking into consideration the cost of the motherboard or any of the other components, I'd say the E6300 is the current king.
Current prices ... on newegg ...
$165 ... AMD x2 4200+
$181 ... Conroe e6300
$220 ... Conroe e6400
The e6300 benchmarks between 0 and 20% faster than an AMD X2 4200+ ... it costs 10% more.
The e6400 is 15% faster than the 6300 ... it costs 20% more.
The price/performance ratio is better at stock speed with an e6300, however, the difference in price/performance is nearly negligable when compared to the AMD x2 4200+ price point and e6400 price point.
Factor in Overclocking, and the e6300 and/or the e6400 would both shine more here than the AMD X2 4200+.
Factor in the cost of motherboards, and the AMD x2 4200+ may have a slight advantage.
But, overall, I'm gonna say the e6300 is the best bang for the buck.