Their plain gray ones are the cheapest and work just fine. (The Nashbar chains are the plain gray KMCs, but you can get KMC-branded for even less than Nashbar if you shop right.)

Since they are so cheap, lately I've been buying the prettier half-nickel plated X.93 series. I use the X8.93, X9.93 and X10.93. The outer links are shiny nickel, and the inner plates, which collect the dirt anyway, are plain gray.