This is my own view, but I feel it is objective and is congruent with the reality of the history of the pure world.
1) FOE - I know this controversial, but FOE has edged out MM for the greatest pure clan simply due to longevity.
2) MM - The face of the pure world, true legends. Nothing else needs to be said.
3) EOP - Another controversial choice, but they have gotten the better of Fatality for the last several years. I also believe EOP's 2010/2011 rank team were true innovators and arguably one of the best of all time.
4) Fatality - Although they're irrelevant now, Fatality in 2012 was the "most dominant clan of all time" as their quality and numbers simply were unmatched.
5) TLP/CP - Could go either way, but I personally rank TLP slightly higher simply due to being a top contender in both servers. CP, however has shown glimmers of brilliance and are the definition of consistent, and have the greatest F2P rank to ever clan (Drunk).
Post your own, try and keep it unbiased.