First of all, best is a rather ambiguous term without knowing what tasks you want it to the best at doing, not to mention if you are going to overclock it.
Secondly, just because it is the best does not necessary mean your motherboard supports it.
Anyone else not bothered that this guy is trying to look for a FIRST GENERATION Intel CPU?
Used first gen prices are not bad, while a new cpu, motherboard, memory, and windows license costs significantly more, Not to mention a Gen 1 i7 is still going to be better at multi-threaded tasks, now for single thread yeah a modern i3 will beat it by a decent amount.
Firstly, I would not recommend staying with an LGA 1366 MoBo, as that socket is relatively outdated. I would recomend upgrading to an 2011 or 1511. Depends on your budget and what you want to do. LGA 1150 is also a good option if you are on a relatively tight budget.
However, if you want to use an LGA 1366 socket CPU, I would recommend looking at pcpartpicker.com and selecting the 1366 socket on the CPU option.