From the Goblet of Fire, Chapter 1 (THE RIDDLE HOUSE):
"If?" whispered the second voice. "If? If you follow the plan, Wormtail, the Ministry need never know that anyone else has died. You will do it quietly and without fuss; I only wish that i could do it myself, but in my present condition...Come, Wormtail, one more death and our path to Harry Potter is clear. I am not asking you to do it alone. By that time, my faithful serant will have rejoined us --"
"One more murder ... my faithful servant at Hogwarts...Harry Potter is as good as mine, Wormtail. It is decided. There will be no more argument. But quiet...I think I hear Nagini..."
What was that "one more murder" that Voldemort was planning and talking about? I can think of 5 possibilities (four actual and one attempted murders in GoF) but none of them fit:
Bertha Jorkins: Was already dead, and her death discussed in that same conversation
Frank Bryce: Doesn't seem to fit, since Voldemort didn't know he was around until after Nagini told him, several minutes later ("Nagini has interesting news, Wormtail," the vice said. "In-indeed, My Lord?" said Wormtail. "Indeed, yes," said the voice, "According to Nagini, there is an old Muggle standing right outside this room, listening to every word we say.")
Barty Crouch Sr: Wasn't pre-planned (necesitated by him fighting off Imperius Curse placed by Jr. and escaping).
Cedric: Wasn't planned - Harry was meant to end up in the graveyard alone.
Harry's attempted murder: The murder spoken of here was meant to be before Harry was in Voldemort's hands.
Also, as Xantec noted, it wasn't Moody, since he was needed alive for Crouch Jr.'s impersonation up till Harry's capture.