I think the general consensus here is the i5-750, but it really does depend on what exactly you want to do with the system.
So, Mr OP, let us know what you want to use your shiny new rig for and the CPU choice will become clear.
In a nutshell though, it seems to go like this:
Purely gaming with a single but powerful GPU: Phenom II X4 (might as well go 955 BE) or Core i5 750
Mid-level number crunching (i.e. fairly complex spreadsheets) and mid-level design/media (graphics, CAD, video): Core i5 750
High-level gaming with multiple GPUs, intense number crunching and top-level design/media: i7 930
All those CPUs can be massively overclocked as well, pushing performance up to the "tier" above.
If budget is very tight, or you're not...