Biggest bang for the buck? Argh! Tough question but - I would have to say the right cables can make a sad system sing and a great system sound utterly magnificent.
The margin of improvement by a single component in any system is far less - regardless of the quality of the component. Dumping a pair of Tenor's in a system comprised of a first generation CD player, radio shack preamp, and cheezy speakers is not going to provide the same results dollar for dollar as if the problems of the system were addressed with better suited cables. How much are the tenors? 20K? That's one heck of a cable budget! We may have money left over for better speakers!
There are many reasons for this - most of which *I* cannot explain. We often read how the electrical properties of cables, the preservation of signal, the capacitance, reactance and inductance all work to make it "better" but focusing on this is like looking at only one piece of the puzzle.
Here is an example. The new BMI Shark power cord is made of platinum. We know Platinum is a crappy conductor but people claim the Shark is an awesome "sounding" power cord. Likewise there are other cables (and components) which measure poorly yet sound very musical.
Why is this?
To get a better understanding of why cables make such a difference, one must first better understand the world around them. Everything is in play. While the preservation of a signal is important - there is truly so much more. Take a look at
HyperPhysics to get a grasp.
Did I say cables can give you the biggest bang for the buck?