That is a really hard question to answer, but I'll do my best.
First thing is first: they are all bad. Really, even Mxyzptlk, when you consider his potential for power in beating back the wrath of God in the form of The Spectre when he accidentally gave the Joker near-omnipotence in "Emperor Joker."
All of them are bad, but who is the worst?
Now, the answer I'm going is give is clearly subjective based on past stories involving the DC multiverse. I believe the greatest menace to the DC multiverse has been the Anti-Monitor.
When you can have one character who was responsible for such wholesale slaughter and who had killed characters who are still dead to this day, not to mention wiping out and rebooting the DC continuity all by himself, that is power and danger wrapped up in one hideous package. You can bet that when I saw the Anti-Monitor's appearance in the Sinestro Wars that I knew there was a huge danger to the lantern corps with him in the mix.
The very presence of the Anti-Monitor in this universe represents a danger that would require the entirety of the forces of good to battle him. He has personally wiped out so many earth dimensions that we cannot look at him without understanding what kind of game changer he is.