I would have to say that mine is my farseer.
I have long imagined a list with 2 autarchs or even a phoenix lord and no farseer, and what a bleak list is it! Farseers just add so much spice to an otherwise bland game.
A farseer just instantly adds an extra layer of complexity as with the help of a spell or 2 dire avengers can stand up to terminators in cc (5+ rerollable invul) and howling banshees can kill a squad of marine on the charge (doom!!)
The ability to stand back and manipulate the tide of the battle is just something so appealing to me in an HQ that game wise and fluff wise I cannot picture an eldar army without a farseer.