Third worst ever: I am actually lowering the ranking of my previously-most-confusing prologue of Crystal Dragon. This is now coming in third. While I still think it is the most confusing prologue to start a book with of all time--to the point where I thought I had purchased the wrong book by mistake for a good chunk of pages--at least, by god, the prologue material is actually used in the story later on, and by later on, I mean "before the dead end of the book."
Second worst ever: A Feast For Crows wins for literally taking as long as possible, i.e. the dead last sentence of the story, to get back to whatever happened in the prologue. To the point where I had long since forgotten about the prologue, since it was 1000 pages ago and I no longer cared. I say that as a speedreader who took a week-ish(?) to finish the book, rather than the months it probably takes other people to read it, so I had a better shot at remembering that stuff than most people. And I still didn't. No longer caring or remembering what happened in the prologue by now pretty much destroyed all shock value that the author had been going for. Buzzkill.
And now, for the WORST PROLOGUE EVER! I honestly did not think it would or could get worse than the prologue not being referenced again until the dead end of a book, but somehow George R.R. Martin managed to top himself by never, ever, ever referring to anything in the prologue of A Dance With Dragons ever again, even at the dead end of the story. This made the prologue utterly fucking pointless to the entire rest of the story. Plus the prologue didn't really tell us anything particularly important about warging that we hadn't already/read figured out previously with other characters. It just seemed to be there for starting out the story with creepy gore and that was it. FAIL.