This was a difficult one. There are loads of items clogging my bank that mean something to me, as well as actual characters, places and so on.
So, I eventually concluded, after failing to narrow it down to something clever, that I would go a different route and think about it more from my character's perspective than my own, though what I have chosen appeals greatly to me anyway because of the lore implied.
I opted for the Circle of Stone in the Jade Forest, where, once you complete the quest Family Tree (a real shame that the Alliance can't do this one), your characters 'ancestors' appear in a ghostly, smokey haze. I actually thought this was an incredibly interesting quest, because players who are unfamiliar with the lore and history of Warcraft may not realise that Humans are descended from Vrykul, that Blood Elves are descendants from High Elves, kin to Night Elves, and may simply overlook the fact that Forsaken are not born undead, and used to be Humans (the playable Undead, anyways). So this is a quest that I look forward to on every character, regardless of whether I've already completed it for my race or not. Even though these are just random NPCs, I do feel a really silly kinship with them through my character. So that's why I've chosen this as Day 6.