I am going to harbor a guess and say that the most powerful swordsman will all collectively be found in Rokugan with each clan producing a uniquely powerful swordsman that can match any other clan's swordsman - all simply have different abilities.
As a prelude to this example let me first say that the reason why you don't see threads on the most powerful swordsman is because melee combat is by the nature of D&D so much more diverse than archery.
Archery by definition is limited to bows with a clear hierarchy to the power of bows with a mighty (+4) daikyu (composite long bow that you can kneel with) being the best bow out there.
Since every munchkined archer then will wield a daikyu, it is simply a selection of feats and classes. Here is a non-inclusive list of the archery feats:
point blank shot
rapid shot
precise shot
ranged shot
shot on the run
sharp shooting
zen archery
snatch arrow
mounted archery
psionic shot
the deeper impact version for ranged attacks (IA-IDHMBWM)
many shot (the non-epic feat in the epic handbook - again - IDHMBWM)
the dragon feats - let you do ranged sunder, ranged disarm, maybe two others - I forget
then you have
WF, WS, IC, PC (not sure if that applies to ranged weapons)
Rokugan has Way of the Wasp, WotS has Sting of the Wasp
I think that is all of them by official DND sources (as well as Rokugan stuff)
With regards to archer PrC's you have PA, DWS, OotBI, Bounty Hunter from Rokugan, and Tsuruchi ______ from WotS (sorry forget name), and a Unicorn PrC from WotS that focuses on mounted archery.
The melee combatant who lets say focuses on swords has scores of swords to sort through each with its pros and cons with no specific sword being better in all sword fights than any other - as opposed to the Composite Long Bow being the obvious choice for an archer.
The Swordmaster also has about 10 to 15 times the number of feats to choose from - IOW - it is simply impossible for the swordmaster to take all sword related feats as opposed to the archer who can get just about all of them.
Finally, the number of PrC's out there for a swordsman, to put it mildly is a bit daunting to sort through. There are some that stand out above others but there is no one particular that is the best since they are all focused to some degree on different elements of sword play.
Basically, if you want to pick a swordmaster that is "the best" you have to choose a sub speciality.
If I may commandeer this thread a bit, a propose to first limit this contest to characters who would preferentially select a sword (any/many/one/all) over other weapons. A frenzied berserker, as alluded to above can certainly dish out the damage. However, they are a lot more effective (well actually, in any normal game, they are just overkill and they can use their barehands, but that is another matter) with a weapon that has a high crit modifier like an axe or pick/scythe and power critical but more importantly are
VERY effective with any weapon. An iaijutsu master seems to mesh better with my sense of a "sword"master than does the frenzied berserker (IMHO) because they focus completely on the katana and their abilities augment the use of that sword.
If you all agree to this limiation I next propose the following categories:
Best overall - this would include all abilities - probably going to wind up being a swordmaster that is a bit more diverse at the cost of alot of power in one specialty
Best defensive
Best offensive
Best offensive in first round
Coolest
Again, I challenge you all to find better swordsman than are in Rokugan.
THe best defensive would definitely goto the Phoenix Heaven Wing's initiate with soul of the four winds and a maxed out Void (using Depths of the Void). This guy's AC is somewhere around 100 - 120 and unlike super AC man, he has tremendous (I mean PHENOMENALLY TREMENDOUS offensive power) - He will be able to get something like a +80 bonus divided between any number of attacks, damage rolls, saving throws, armor class boosts and can do replenish that +80 once an hour.
Best offensive (in the first round) would goto the Iaijutsu master - simply scary with two attacks equating to a standard action, the bonus feats, the free surprise round, AND THE CHARISMA BONUS TO
EVERY DIE OF IAIJUTSU FOCUS. As someone mentioned above - this guy can do something crazy for damage - in excess of a 1000 points in the first round. With expert tactician, he is unstoppable.
Best offensive in general probably goes to the Lion Singh Ragers - the full attack at the end of the charge is what will do it combined with greater rage, a raging weapon (from OA), and their crazy ass cool power attack feats (one lets you still hit if you subtracted too much with PA, the other lets you get a bonus to your next attack if you subtracted too little).
There are a few others that are pretty cool for overall effectiveness. The weapon master being able to do whirlwind attack as a standard action is damn cool - combining that with spring attack is extremely dangerous because all of the downsides of using whirlwind attack are negated.
I would argue against the duelist since the extra 3d6 max damage can't be multiplied. The ghostwalker is damn near the top for cool factor or overall effectiveness. However, this requires far more in game role playing for this to work - go figure

. The Master Samurai is pretty good (great for the Unicorn samurai from Rokugan), but the mounted combat req's suck. The tempest is definitely a powerful class considering they get 8 attacks instead of the normal 4. A tempest with 2 speed weapons (10 attacks) and pumping all his energy into getting bonuses to hit is very dangerous.
However, I would have to argue that the coolest swordsman is either the Mirumoto Niten Master (OA) or a straightup Scorpion Samurai - the disarming capabilities of the scorpion are simply AMAZING. Combined with feats in Song and Silence, they become very deadly opponents. The Niten Master gets a good AC, good attack, can do some magic stuff, gets spell resistance, and combined with the Mirumoto Elite guard gets mettle, evasion, the ability to prevent the use of spells, and spell like effects and with the right kata can cast dispel magic.
Guess this turned into a bit of a pitch for Rokugan - sorry about that - its just the world is so damn cool.