In the opinion of some manufacturers with extensive test facilities that I know, the KMC Z8 gives the most kilometres per euro spent. Other manufacturers, like Thorn, think the KMC X1 is the best, period; I'm not aware of any forum member yet wearing one out, so we don't have a relative miles per pound number. My own opinion is that the X1 commands such a premium that the X8 or Z8 will prove to be much cheaper when we can compare miles per pound.
I've had the KMC Z8 and it is first class; in one bound I multiplied my mileage by between two and three times over the inexpensive Shimano chains I used before. The problem is that most mailorder merchants sell so few of the Z8 that they don't get much of a discount to pass on. Compare the price of the derailleur equivalent KMC X8, of which almost every dealer sells a ton, and therefore gets a huge discount which he can pass on to customers. So I buy the X8 on sale and put them by.
I'm impressed by the X8 but I am, and have always been, very heavy on chains; if you're a spinner with a light touch it is possible something cheaper from the KMC line may be slightly more economical for you. I have concluded that a derailleur chain, because of the flex required, is probably ipso facto a better chain than a single-speed chain of otherwise similar materials and construction.
There is an extreme experiment in running an X8 on the factory lube alone, no added lube, at
http://thorncyclesforum.co.uk/index.php?topic=6813.0 -- note that those mileages, which by themselves may not seem much chop, are between two and three times further than anything I ever achieved before on other chains.
