Most PvP achievements are my bane, as I've come to realize I can't fight players as well as world/dungeon mobs. Maybe it's something wrong with my brain and reflexes, or I have some inner struggle against player to player conflict, much like I have IRL. Arena seems a little nicer because no one is there to make me feel bad, if I'm with friends. So I guess any of the battleground achievements are the ones I hate the most. I can't exactly pinpoint one.
If I don't count PvP, since it literally doesn't exist to me because of my ineptitude, then my most hated achievement would be the yak mount. Too much gold. Again, maybe that's my problem.