Depends.
i7 980X is the new 6-core and is the best you can get, but it costs $1,000 and will be a waste unless you're using heavily-threaded apps like 3D modelling, design and video production.
i7 920 is probably the best i7 all round because it can overclock just as well as the more expensive versions.
Be mindful though - the i7 860 is actually for socket 1156 motherboards, whereas all the i7 900s are for socket 1366.
But, we go back round to the original question you asked - unless you're doing intensive multi-threaded work you'd probably be better off with the i5 750.