These are from the date when it was possible to begin working on them, or the previous hard mode was killed.
Not to suggest this is indicative of their relative difficulty but I find it pretty interesting.
Brown for Vanilla. Green for Burning Crusade. Blue for WOTLK. Red for Cataclysm.
#1 Ouro - 87 days from the Twin Emperors' death. 26th April 2006.
#2 C'thun - 86 days from the Twin Emperors' death. 25th April 2006.
#3 tied Chromaggus - 74 days from Ebonroc's death. 25th September 2005.
#3 tied Ragnaros - 74 days from Majordomo Executus' death. 25th April 2005.
#5 Yogg-Saron, Alone in the Darkness - 70 days from One Light in the Darkness. 7th July 2009
#6 High Astromancer Solarian - 59 days from Magtheridon's death. 24th April 2007
#7 The Four Horsemen - 56 days from Gothik's death to D&T's kill. August 25th 2006
#8 Al'ar <Phoenix God> - 48 days from Magtheridon's death. 13th April 2007
#9 The Lich King (Heroic 25) - 42 Days from Heroic Putricide's death (first pull). March 26th 2010
#10 Heroic Al'Akir - 36 days from Heroic Conclave of Wind's death. January 22nd 2011.
#11 Kael'thas Sunstrider <Lord of the Blood Elves> - 31 days from Solarian's death. 25th May 2007
#12 Magtheridon - 29 days from the first 70 raid kills. 24th February 2007
#13 Lady Vashj <Coilfang Matron> - 17 days from being unlocked. 29th March 2007
#14 Leotheras the Blind - 16 days from Hydross' death. 4th March 2007
#15 Loatheb - 14 days from Heigan's death. 17th July 2006
#16 tied Heigan the Unclean - 13 days from Noth the Plaguebringer's death. 3rd July 2006
#16 tied Hydross the Unstable <Duke of Currents> - 13 days from Gruul's death. 16th February 2007
#16 tied Heroic Cho'gall - 13 days from Heroic Ascendant Council's death. 15th January 2011.
#16 tied Heroic Nefarian - 13 days from Heroic Atramedes' death. January 9th 2011.
#20 tied Archimonde <The Defiler> - 10 days from Azgalor's death. 9th June 2007
#20 tied Gothik the Harvester - 10 days from Instructor Razuvious' death. 30th June 2006
#22 tied Mimiron Firefighter 25 - 8 days from Ensidia's KnockKnockKnock kill to Firefighter kill. May 1st 2009
#22 tied Sapphiron - 8 days from Four Horsemen dying. 2nd September 2006
#22 tied Gruul the Dragonkiller - 8 days from Maulgar's death. 3rd February 2007
#25 Reliquary of Lost Souls - 6 days from Gurtogg's death. 2nd June 2007
#26 tied Kel'Thuzad - 5 days from Sapphiron's death. 7th September 2006
#26 tied Kil'jaeden <The Deceiver> - 5 days from the opening of the third Sunwell gate. 25th May 2008
#26 tied Sinestra - 5 days from Heroic Cho'gall's death. January 20th 2011.
#29 tied Heroic Anub'arak 25 - 4 days from Heroic Twin Val'kyr's death. September 6th 2009.
#29 tied M'uru. 4 days from the opening of the second Sunwell gate. 4th May 2008
Thanks to SK-Gaming's handy list of all time world firsts and wowprogress.com.