Kite (1998)
This is a strange one. I had only heard of it because I watched the spiritual sucessor which I only found while looking for abnormally well-developed or beautiful hentai from the 80s and 90s. Despite this it seems to have been good enough to inspire things such as Psycho-Pass, Kill Bill, and even getting a live action remake with Samuel L. Jackson.
Off the bat I should say that this is porn, or at least published by a porn studio and has stuff in it that could be considered porn. It's better summed up as a weird action drama amalgamation with insane 90s styling direction and brash adult themes. TLDR there's a lot of factors that take precedence over the pornographic part, even though it's still very present

The protagonist, Sawa, secretly works for a cop, Akai, performing assassinations that Akai later investigates the crime scene of in order to hide eveidence. Sawa's motivated because her parents were murdered when she was very young, and Akai had taken her under his wing while she trains to ultimately get back at the person that killed her parents. It's known that Akai was the one that murdered them all along, and he'd been sexually abusing Sawa as part of the deal.
This is a pretty good premise for what it is, especially only being an hour long. I do need to say that the sexual abuse is fully animated, though all but the last scene of which can prolly be skipped without missing any plot, as long as you know that it's there. And it pays off (spoilers) when she fuckin shoots him directly in the dick near the end.

The action scenes are extremely well animated especially for something published under a hentai studio, was mildly disappointed to see that their lineup is mostly only generic hentai slop otherwise. The quality looks like it mainly comes with director, and they've done other work with things like robot carnival.
Getting mildly off topic though, Sawa uses a handgun with exploding bullets, which results in some super sick close combat bloody action scenes. Minor spoilers but my favorite one is when they fall out of a building while fighting, and they land on a car, which crashes through a bridge, which crashes to the ground, which crashes through the ground to a subgarage. It was so over the top out of the blue and it literally left my mouth agape.

It probably helps that it's so short but the character design is also really good. Even their targets and side characters have rather unique designs. I also haven't noticed it mentioned anywhere else but one of her outfits seems to've been inspired by evangelion. Also there's a guy who's name is pronounced "Kanye" and he looks like this.
Lastly I do gotta say the art is general is beautiful. I would go as far to say that it's a staple of 90s animation.

While I wouldn't reccomend this to everyone, if any of the above intrigued you I'd highly suggest watching it. I think(?) there's a cut that removes a lot of the sex scenes too

Inspired by Megrim Haruyo. While not rearly as good, it was fun to experiment with an aliased multi-layer pixel technique