What is Requirement to Unlock [Unique Skills]? According to Kawahara Reki there supposed to be 10 [Unique Skills] on Sword Art Online. I know few of them.


Dual Blades = Player with Fastest Reaction speed.
Holy Sword = Strongest Player (GM giving himself the strongest player title)
Darkness Sword = Highest Player Killing count.


All [Unique Skills] should be rewarded after Floor 90. What are other [Unique Skills] and requirement to unlock it ?
Up until now, only 2 [Unique Skill] have been mentioned in the story:


二刀流 (Dual Blades): fastest reaction speed
神聖剣 (Holy Sword): strongest player


However, according to a web-anquette interview (cannot find the original source),


  Q...他にユニークスキルなど考えてましたか？
  A...90層以降、更に８種のユニークスキルがアンロックされる予定だった。『暗黒剣』『抜刀術』『手裏剣術』『無限槍』など……デザインに苦労したんだが……。
  
  Q...Have you thought about the other [Unique Skill]?
  A...Above Floor 90, 8 more [Unique Skill] have been planned to be unlocked. Examples are (see below), but there were troubles about the design...
  
  
  暗黒剣 (Darkness Sword): highest PK count (if-situation at floor 95, Kirito vs PoH)
  抜刀術 (Swordsmanship)
  手裏剣術 (Shuriken Technique)
  無限槍 (Infinite Spear)
  


Note that since they are still in planning, the real [Unique Skill] might be different.



References:


SAO Fan Wiki (Japanese)
Japanese Yahoo! Answers: 1, 2, 3
Arcadia (Japanese BBS)