While Countdown was probably the STUPIDEST thing ever published, I have to go with Ultimatum being worse. It ruined the best thing Marvel had going for it with way over-the-top gore and just brutal, senseless killing-for-the-sake-of-shock-value. I don't care how they try to spin it; it IS killing for shock value. And that's meaningless and a bad idea.
At least Countdown's problems were mostly (MOSTLY) just OOC moments, bad pacing and horrible continuity/editorial problems.
Jeph Loeb just needs to stop. I can't think of one good thing he's written in the past five years. I feel badly for his personal problems, of course, but maybe he needs a sabbatical or something.