BB King from World of Warcraft. You need to get to the leader of each race in the opposing faction and hit them with a BB gun. The BB gun does no damage and does not auto-hit, you need to get it from a Christmas present with a very low chance of dropping it, and you can only get it during 2 weeks of the year.
Alternatively, "The Insane" from the same game. You need to raise reputation with several factions, 4 of whom you will lose rep with by gaining it with one of the others and have few ways of getting rep from. Before Cataclysm came, you could expect to spend 360 hours on getting it. Access to a rogue was also non-optional. Not sure what it's like now, but it should still take a long time.
Alternatively, "The Insane" from the same game. You need to raise reputation with several factions, 4 of whom you will lose rep with by gaining it with one of the others and have few ways of getting rep from. Before Cataclysm came, you could expect to spend 360 hours on getting it. Access to a rogue was also non-optional. Not sure what it's like now, but it should still take a long time.