Why are there only Tara and Fred in the poll?
Of the two of them (and kinda also even if you add most other dead characters) I'd have to say Fred. I think what made her death so sad was just how final it was. You have the cutie pie of Wolfram&Hart, and all of a sudden she just starts dying away, no one can do anything, and we even learn it's gonna be final, her soul forever destroyed. Also, Tara's death was quick, Fred was really suffering when she died. Her last words ("Why can't I stay?") were really just heartbreaking.
Added to that, I guess it kinda sucked for the Fresley shippers.