lets see, so many good duelers in the past that are gone; do we consider those? I will go through my list of good challenges...
steviefow - super tough; I don't believe I have ever beaten him when attacking him. One of the few soldiers I won't challenge unless at full health =)
Realm of Deceit - one of the other soldiers I won't challenge unless at full health. for the same reasons as Stevie.
Azuk - this guy is a good dueler and high level but I have never had any real trouble with him...
Peddler - as RoD said, this guy is a killer. back in the bygone days I used to dominate him, but at some point he got his tactics up and from then on, I get KOed every time I take a shot at him. Never have defended against him, I don't think he attacks that often.
Josey Wales - my towns version of Peddler; Josey doesn't do much attacking and isn't a pure dueler; but he is a soldier and has his defensive stats allocated in the right way to be a handful. there have been times where damage resistors have KOed me and gone on to get smoked by Josey
bigpopa - another good dueler (from what I have heard) who I have never really had much trouble with... although he is respecing skills now that he hit lvl99 so I would expect him to get much more dominant soon
ag3352 - never fought as we are allies. I remember when ag was respecing to become a dueler and we had several long discussions on what builds were out there in w1. apparently he/she chose the right build
phonebook - we go back and forth. he usually must lose a duel or two to figure out my aim/dodge pattern (much like passis used to) before he can win. I don't have much respect for him as a dueler
fonte - only fought him a couple times; although his alter-ego passis used to be a regular pain in my side
Tuxedo - a good dueler, and high level, but I have rarely lost to him
Johnny Killer - this guy is VERY tough; he has gone inactive (sitting in my town to take potshots at me on occasion lol) and in my opinion is one of the lesser known top duelers in w1
Codect - a hernandos sword wielder who is quite a handful; respectful guy though who is fun to have a conversation with about dueler builds and strategy
ascotia - one of the original (and the most successful) resistance duelers. in his prime, this guy was close to unstoppable. the only way I could occasionally score a win was luck; eventually we would go back and forth on aim/dodge patterns because he knew he had to hit me at least twice to win, since I would hit him every round.