Most powerfull would probably be the new i7 form Intel. Trying to find some benchies now.
Yip, i7 is tops at the moment...... expensive tho. The AMD Phenom II X4 955 does well for what you pay for it.
Firstly the i7 920 isn't that bad price wise and secondly the OP didn't mention anything about band for buck. If someone asked you which rules with power, a corsa lite or a Bugatti Veyron you not going to say the corsa lite owns when it comes to bang for buck because they didn't ask which is best in regards to bang for buck, they asked which owns the other in performance.