If you're going for best mileage you can't do better than OEM.
There are aftermarket parts which are considered OEM-equivalents that might be able to do as good a job, or at least close, but nothing will do a better job on a stock engine than the oems.
Only if you're increasing the air and fuel would higher-performance aftermarket plug wires, coil, etc. actually make a difference. On a stock engine they might not work as well as oem(edit: I'm referring to the coil there).
Only possible exception would be NGK oem-equivalent plug wires (blues, not the reds) which I've read offer less resistance than the oems that came with the car. In theory they might be better for mileage and even power but it won't be something you'll notice while driving the car.
That's my take on it, I'm sure others have different opinions.