For the most part id say I prefer american cartoons.

Transformers, Beast Wars, Adventure Time, Regular Show, Ninja Turtles (I liked all 3 series), Auqa Teen, Johny Bravo, Powerpuff Girls, Street Sharks, Swat Kats, Batman Animated Series, Avatar, Legend of Korra, Spongebob (has recently grown on me), Samurai Jack, Beetleborgs, Magic School Bus, 2 stupid Dogs, Dexters Lab, Power Rangers, 90s Spiderman, 90s Xmen, Centurions, Mighty Max, Inspector Gadget, Scooby Doo, Flintstones, Looney Toons, Tom & Jerry.

But Japan has DBZ and Pokemon so it wins.

I'd say American animation overall because there's lots more variety and style than there is with Japanimation going back to the old Disney, Warner Bros, and MGM / Hanna-Barbera years, aka the "golden age" of animation.

There's alot of American TV shows from the 80s however that are influenced by Japanese-style animation, like TMNT, Real Ghostbusters, Transformers, etc. Rankin-Bass classics like The Lord Of The Rings / The Hobbit and Flight of Dragons also share many similarities with anime style as well.

Then the cartoons of the 90s and especially the 2000s started featuring a more simplistic style to them. I can't stand watching any of the modern kids cartoon shows because they all feature the same washed out pastel colors and bland backgrounds. Love me some Boondocks, but that of course is a distinctly anime-influenced show.

I haven't seen adventure time, but I have seen Regular Show, and I can't see how story, voices, and originality are better in that cartoon than anime.  You clearly haven't seen enough anime if you think that.  

Adventure Time, I would say, looks like it has a good story, is original, has a lot of imagination, and all that.  However, Adventure Time is an exceptional cartoon and is by no means a standard.  There are also exceptional anime.  

edit: 
Watch this trailer and tell me how this lacks all 5 of the qualifications you listed in comparison to American cartoons?

i respect you're opinion, its a joke

Japan. Not so much because its anime/art style/story etc but animators over there (from what I've seen) to an extent, still practice traditional animation. Yeah the movement of the characters, some objects seem really slow, but when the situation calls for it (like an epic battle etc) you can see the animation work beautifully.

If you look back at the golden age of American animation, they were the best of the best; frame by frame @ 24fps. Yeah a lot of them were racist/offensive (hurray for political correctness) but if you can look past that, the animation is where its at. According to Mel Blanc (famous voice artist) animation was pretty pricey (still is for this type). For the 1940/30s, $50K was a lot.

Now FF to today most cartoonists have relied on CGI and Flash animation too much IMO. I my honest opinion, I see very little effort in cartoons made this way. They look like paper cutouts moving on a flat surface. Keep in mind I'm not saying that I hate this type of style, but this style (along with CGI) has caused a saturation.

I loved cartoons like Hi Hi Puffy Ami Yumi, but even then there weren't that many like today.

well if you are going to count full fledged cartoons. Animated movies. America by a landslide.

Not exactly sure what you mean...

But I prefer Japanese anime because of the deeper plots and character development but I do like American cartoons a lot and I watch them more often than Anime. It's about 50/50 but if I had to choose I'd pick Japan

Depends on what you're looking for (from an American perspective).

Comedy from Japan is hit or miss, especially since some of it doesn't translate all that well. Action is usually better, though your mileage varies much more with the budget. Some good examples of drama, since there is a strong market for adult-focused japanese animation. Artwork tends to be stronger overall, though likewise very dependent on budget, and is more than a little homogenized.

Cartoons in the US usually (with some exceptions), are geared more towards kids, and don't tend to have a budget unless (or sometimes especially if) they are a brand. As such, most are pretty unsophisticated, and those that aren't tend to be comedies. Few dramas. Animation almost uniformly minimalistic, though a greater variety between studios.

In terms of movies, I'm tempted to rule for the US. Part of that's probably a lack of exposure on my part, but Miyazaki, who gets perhaps the most recognition, I find decidedly hit-or-miss.

In terms of non-feature animation...Japan. The US has some great series, but there simply isn't enough variety. There's a fair degree of sameness in the Japanese market as well, but there are also a lot of standouts.

I don't know but I guess Japan. America has some good shows too so I actually don't know

American artist have caught up to the lead Japan had. Japan has seemed to hit a brick way in terms of unique styles and progression. The last great noticeable step was mixing in cgi into animated movies but even that was rushed, in early work the cgi looked sloppy, rushed and out of sync with the entire scene. Western studios have adopted that as well but since the technology is more refined you see those additions and they feel like an improvement rather than an eyesore.