Apr 16th 2010 7:40AM I know this isn't really an achievement, but getting the Grand Marshal/High Warlord rank back in the days was one of the hardest things WoW have ever offered.
It was insane how much time and effort you needed to put into this.
Personally I stopped at rank 11 (Commander/Lieutenant General) after 2 months of constant PvP every single day to keep my ranks up, and getting the Black war tiger as reward.
After that a new task had begun. Getting exhalted with Darnassus just to ride it as a none NE, which wasn't hard, but took quite some time to gather all the runecloth that was needed.
A small bonus from this was that you got exhalted with all the BG's and thereby Justicar.