Highschool of the Dead is an anime that in my opinion can be extremely enjoyable if you watch it with the right mindset. No it doesn't have a great story, the characters aren't anything you haven't seen before either, but what it does deliver are some great fights, music and overall atmosphere and that's what you should be expecting going into this show.
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Art & Animation:
This show is surprisingly well animated and the art is pretty great as well, which isn't all that common for a harem anime. A lot of scenes honestly look incredible with fluid animation and direction, especially the fights and I only found out it was made by MADHOUSE after my 5th re-watch or so, which definitely explains why the visuals are so good.
Characters:
As I said before, the characters aren't anything special, but they are pretty well written considering that (let's be real) they were probably more of an afterthought and they actually get shit done. Every character, yes even the MC, has an actual personality that even somewhat evolves throughout the story. Unfortunately the anime will never get finished so some characters don't get developed as much as others but I'd say that isn't too much of an issue as everyone still has fairly similar screen-time.
Story:
The story is fairly simple and about as complex as you'd expect. All of a sudden a zombie apocalypse breaks out and they're trying to survive and find their family. It's a pretty generic zombie plot but that's one of the reasons I like it so much, it's not trying to re-invent the wheel and the anime medium is lacking in the zombie department anyway. Occasionally some characters get their own short arcs here and there with a little flashback or something to develop them further, which is a nice way to change up the pacing a little from the pretty mindless gore and fighting (which I love don't get me wrong). One unique thing that comes to mind is how the zombies are completely blind and only respond to sound which does set this show's zombies slightly apart from others' and makes for some unique scenarios.
Sound:
The music is probably one of the best parts about the show, especially the opening which is one of my favorite songs of all time. It always fits the scenes and really adds to the atmosphere, it really never feels out of place. It also has a unique ending song for every episode which is quite uncommon and the only other anime I can think of that does the same is Chainsaw Man. The english dub is also really funny and this is one of the few shows where I'd actually recommend the dub over the sub.
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Summary:
There isn't really much else to say so why do I like this show so much and why did I watch it 7 times? It's mindless fun that's really all there is to it. A simple story, fun characters, great visuals, atmosphere and music just make for a really fun show. Highschool of the dead is an anime that doesn't try to be anything that it isn't, and it manages to be entertaining with it's stupidity and simplicity while getting the basics of what makes a zombie show fun just right. If you wanna watch something that doesn't require too much of your attention while also being extremely entertaining, this is it.