I'd be surprised if Familiar wasn't #1, it's the best card out there for delivering multiple curses to your opponent's deck in 1 turn and you ignore that at your own peril. If you ignore familiar on the board, you have to either be willing to lose the curse war or be able get a head start on the cursers. If you've got an deck that can shrug off curses (Jack, Ambassador, Masquerade), I'd imagine that you could do the first but it's hard to beat familiars in a straight race between cursors. The reason is that the number of curses in your opponent's deck is a product of how quickly you can cycle your deck and familiars don't collide with each other and help cycle your deck.
That doesn't mean I think they're the best attack card ever, I would certainly take young witch and sea hag over them and ignore them completely if IGG was on the table but they're still a guaranteed curser and you can't ignore guaranteed curses on 90% of the boards.