That's such an arbitrary comment. It's not about whether or not a show "beats" another. The question, given by the title of the thread, is "What are the most successful animated shows that featured horror?"
I think Invader ZIM featured horror in the form of weird science fiction in the traditional sense. It's a good show, but it's not really successful.
The thing is success differs from person to person and people confuse popularity for success. Popularity doesn't always equal success (though in some cases, it does, but when popularity fades, the success isn't there [see American Idol, Survivor, Millionaire, Deal or No Deal, Weakest Link, et. al]). Success is marked by the fans and viewers, the attention it grabs, its impact during and beyond its run both locally and worldwide, and a combination of any of these. Success is a strange concept because what may be successful to some people isn't successful to others.
Under the definition of success I gave, I don't think Invader ZIM was successful. The series has a fanatical vibe about it, only attracting a select few fans, but only connecting with them and not beyond them. You can't totally blame Nick for the show's lack of success as so many does. It was just made for the wrong audiences.
Some of the shows presented here are popular, but not successful. Shows like Gungrave, Requiem From The Darkness, and Paranoia Agent are popular, but their success is limited to a select group. Same deal with Courage the Cowardly Dog, Yu Yu Hakusho, Ghostbusters, The Real Ghostbusters, Count Duckula, and The Grim Adventures of Billy and Mandy. Something like Death Note or Bleach has the potential to become successful in time provided they get into the mainstream. Time determines whether a show's success becomes a reality.
And only one horror-influenced animated series (not animated movie nor short) has been seen as a proven success, and, again, that show is Scooby-Doo. It has been around for almost 40 years, imitated and duplicated, and definitely created an impact beyond the medium it was created in.
Th