For some of you it would probably be an item of sentimental value. The dented stick you started the game with. Or the cat your guildmate left you before cancelling their account. Or the purple that dropped after the 18th wipe at 4 in the morning.
But for me, the one item I could never part with is the one that makes me the most powerful character in the game: my Light of Elune. It's level 25 quest reward out of Ashenvale and it grants immunity from all damage and spells for 10 seconds. The number of encounters that this item would render trivial is endless. Except for one little thing: I can never bring myself to actually use it. Because if I do, then I'll never be able to use it again. Twisted logic, yes, but it has me in its irrational grip and will never let me go. And, thus, I can never use it, but I can never give it up.
Now I leave it to you, dear readers, what is your precious?