I'm don't know too much about the current standings, so I'm basing my list on what I personally experienced (mostly in clanwars) between 1999 and 2006. The list is somewhat in chronological order; of when I believe people attained their 'peaks'. Note that I've never really been into duels myself so my list is *mostly* teamplay based (the exceptions probably are FireSworD, BunnyElmer and Vegito, whom I included mainly for their dueling skills).
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Dizzy: for reasons already stated by Urbs, although I never got the pleasure of playing a clanwar against DL or DM when he was at his peak.
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FireSworD: every time I played against him in the old days, he was a very powerful opponent. Granted of course, that he had some decent teammates on his side, because not everyone knows how to adapt to his playstyle. But in duels, his strength was indisputable.
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CelL: when FoR beat CC in 2003, I remember him as being the strongest player on their team. In subsequent 2vs2s/3vs3s in the months thereafter, he remained a strong player.
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BunnyElmer/Anakin: in the few years before we permanently 'joined forces' in 2004, I've always considered him to my biggest rival (in a friendly way) when it came to competitive play and leading a clan in general. He truly left an impression on me, even though few would remember him today. One of the "Battle1 legends".
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Grytolle, Vegito and myself: obviously, this is based on our performance in JJ2WC2 in 2005. I don't think any other team has ever put so much effort into preparing for a tournament as us. We really got *a lot* better in a relatively short amount of time, and finally put our clan on the map. I know it may sound arrogant that I included myself here, but I honestly don't think I contributed any less to our victories than my teammates.
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Superjazz/Vivando: in the time to come after JJ2WC2, the new clan

quickly rose to become CC's newest rival. Although I have respect for Yasco's and Squirrel's teamplay, I must say that Vivando was their strongest player overall. He's always been a dangerous and worthy opponent in my experience.
I'm not sure who else should be on this list, so I'm leaving #9 and #10 open for now. I'll give it some more thought. Some candidates are
Maverick,
Skull,
Snooze,
Salamander,
Urbs (because he wants it so much

),
Tidus,
Nazgul,
FawFuL or perhaps
Yasco.