In any case, whether or not DU-style conspiracy theory deserves a place in the historical Hall of Fame, I think it is very likely that al Qaeda fits the pattern identified by Pipes - it's a real-life counter-conspiracy motivated by an erroneous conspiracy theory, in this case the "conspiracy against Islam", a concept which first became state ideology in Khomeini's Iran.
I'd also like to see someone write a study of conspiracist thinking in the world's intelligence services. Once again, it's a situation where "if you're not paranoid, you're crazy", and the challenge is to develop a rational paranoia...
I have this notion of my own that the world has experienced four (and maybe now five) great outbursts of this symbiosis between conspiracism and counter-conspiracy in the past century: 1920s Russia, 1940s Germany, 1960s China, 1980s Iran, and, just maybe, 2000s USA. One might hope that this latest iteration is a little gentler in its outcome, being based more on DIY Internet theorizing than on demagogic mass media.
For a country to be on your list, as I understand it, that country's government must make its major policy decisions based on indefensible conspiracy theories. Is that what you meant?
The list itself is of interest. Cambodia under the Khmer Rouge would seem to fall squarely with the others, and perhaps even to be the epitome of this human failure.