Other than Berserk. And that's a strech
I'm sorry, but Berserk a stretch for a mature anime? How exactly is it a stretch?
Here's a few off the top of my head. Clearly some are better then others (since I have no range of your taste, I can't narrow it down), so you may want to look into them a bit before diving in. I'll provide VERY BASIC plot outlines.
I'm skipping fansub releases, or else I'd be listing stuff all day. These are all available on DVD, however some are in the process of being released, or will be released soon.
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Battle Angel aka Gunnm (OVA series) - Dark story about a cyborg girl and her world (not nearly as cheesy as I made it sound)
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Blood: The Last Vampire (single OVA) - Fairly weak plot, but very dark and mature.
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DevilLady aka Devilman Lady (TV series) - Horror/action with a bit of sci-fi tossed in. Battles are fairly bland due to TV quality animation but the sheer creepiness of the show makes it worth watching and it has a truly fantastic ending.
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JoJo's Bizarre Adventure (2 OVA series) - Not the strongest show plot-wise, but it more than makes up for that with great characters and some very interesting, clever battles.
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Macross Plus (OVA series and movie retelling) - Considered by many to be the best thing to ever come out of Macross. The Macross universe is only used as a backdrop for the mature love triangle story (not your typical cheesy anime love story either) which features some of the best dog fight scenes ever animated.
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Memories (movie) - 3 unconnected stores in one movie. 1st story is an Alien like sci-fi piece (minus the alien), 2nd story is a silly little dark comedy, and the 3rd is an semi-abstract piece about a city at war. Very interesting with some top notch animation.
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Princess Mononoke (movie) - One of many movies directed by Miyazaki. This is probably the most mature of the batch, though all are worth watching.
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Read or Die (OVA series) - Short OVA series with a huge budget resulting in some great action and memorable characters.
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Record of Lodoss War (OVA series and a TV series) - Classic fantasy series. If you're into Lord of the Rings, check this out. The semi-sequel TV series can be skipped, but watch the original OVAs.
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Samurai X aka Rurouni Kenshin (TV series and 2 OVA series) - Samurai series with characters lightly based on real historical figures. Skip the TV series, but watch the prequel OAV series and if you enjoy that, watch the second OVA which ends off the whole thing. This is probably your best bet for maximum mature enjoyment.
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Ghost in the Shell (Movie and TV series) - Sci-fi series. The movie is very mature, but I find it a little bland due to slow pacing and over use of long dialog scenes. The TV series is a mix of very good one shot episodes and a longer continued plotline.
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Ninja Scroll (movie) - Bloody ninja action. Not hentai, but it does have some sex.
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Patlabor (OVA series, TV series, 3 movies) - An older work from the director of the Ghost in the Shell movie. Skip the OVA and TV series as they are more comedy based, but the 3 movies (I haven't seen the 3rd) are very intelligent suspense/sci fi stores with an excellent blend of very realistic action and character driven dialog.
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Riding Bean (single OVA) - This may not have the look you're going for, but it's a fun little action show. Take any classic 80s action hero character and toss him into the car chase from the first Blues Brothers movie and you have Riding Bean. It's a little light hearted, but it's still not a kids show.
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Twilight of the Dark Master (single OVA) - Not the best show, but it's dark and moody sci-fi/horror. Worth a rent.
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Vampire Hunter D: Bloodlust (single OVA) - From the director of Ninja Scroll. Story about a half vampire-half human who hunts vampires. Based on a novel.
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Wicked City (movie) - From the director of Ninja Scroll and Vampire Hunter D: Bloodlust. Sci-fi/horror story about a demon woman who teams up with a human cop to save the world and all that. Like Ninja Scroll, it has sex but it's not hentai.
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The Hakkenden: The Dog Warriors (OVA series) - Based on a series of classic novel series. Various samurai of different class are all connected by a mystical force. Very good stuff with different directors/art styles though the series to keep things fresh.
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The Venus Wars (movie) - About a war on Venus, go figure.
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Wings of Honneanise (movie) - An excellent story about alternate world's quest for their first flight into space. I highly recommend this one.
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A Wind Named Amnesia - A boy awakens into a world where everyone else has forgotten even the simplest tasks, such as speech - only not is all as it seems. This is an unusual sci-fi movie. If you want something a little different, here you go.
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Perfect Blue (OVA) - If Hitchcock made anime.
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Black Heaven (TV series) - A man is chosen to save the earth from an Alien attack with cheesy 80s rock. Not the best show ever, but it's done in a pretty interesting way, and it has a very original premise.
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Blue Submarine No.6 (OVA series) - Man floods earth, earth wants him dead. Etc.
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Grave of the Fireflies (movie) - The horror of war from a child's perspective (there is however, nothing childish about this movie)
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Gungrave (TV series) - Berserk meets the mob.
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Mermaid's Scar (OVA) - There's a kid in here that'll change the way you look at anime children for the rest of your life.