atm? The Fearless for pve.
For pvp most is a grind or people wintrading / playing 100 games in the first 2 weeks before being afk for the rest of the season with 2.9k + rating.
Hardest "achievement" in wow regardless is world first end of tier raiding (10 and 25 seperated), there's a world first 10man and a world first 25man.