Hellsing, Darker Than Black Season 2, and Drifters.

I need a fourth anime composed by Yasushi Ishii already.

Gonna keep it to 1 per composer, otherwise this list would be full of Kajiura Yuki and Kensuke Ushio.

• Garden of Sinners: Kajiura Yuki's magnum opus in my opinion. Great themes, awesome atmospherics. It wouldn't be Garden of Sinners without her music.
• Attack on Titan s1-3 : Sawano's magnum opus imo. The definition of hype soundtracks for me. Never fails to get me fired up during the show. It's a shame that he was replaced for the Final Seasons.
• Liz and the Blue Bird: I dearly love this style of Kensuke Ushio combined with the amazing orchestral piece that the band plays. Kensuke brings such a fragile sound that works really well with the themes of the anime and then the orchestral piece ties the entire thing together. I can't help but cry during the oboe solo.
• NieR Automata ver 1.1a: cheating a bit because I love the game's soundtrack so much and they pretty much straight up used it here.
• Ghost in the Shell (1995): I don't think it needs much explanation.
• Violet Evergarden: Evan Call's use of wind instruments is incredible in this soundtrack. It works so well.

Honorable mentions:

• Monogatari series: soooo many amazing themes
• Chainsaw man Reze Arc soundtrack: Kensuke Ushio again. Mostly because of the pool scene. The rest is really good too though
• A Silent Voice: another Kensuke Ushio one. That one scene... Fuck dude
• Apothecary Diaries
• Terror in Resonance: mainly for the tracks with Arnor Dan
• Madoka Magica: Kajiura Yuki
• Fate series (ufotable run): Heaven's Feel in particular. Incredible work by Kajiura Yuki again. My main issue with Zero and UBW soundtracks is that the theming isn't that strong.
• Fate/Stay Night (Deen version): for all its faults, the soundtrack is actually quite good and is kinda why F/Z and F/UBW have gone down in my ranking for soundtracks due to the themes.
• Frieren: Another good Evan Call one, but Violet edges this one out because it has stronger themes imo.
• Land of the Lustrous: great arrangements. Using different arrangements for different groups isn't necessarily new, but it's executed brilliantly here.
• Clannad: the dango theme never fails to make me cry
• Cowboy Bepop
• Your Name
• Yuru Camp: I love the simple and relaxing guitar soundtrack

Cowboy Bebop!

https://archive.org/details/Cowboy-Bebop-OST-Collection

Some good suggestions here already. I'll add FLCL to the mix. The Pillows' youthful, energetic, and sometimes wistful punk rock fits perfectly with the show's themes of childish rebellion and the pains of growing up.

Samurai Champloo by a country mile.

Runners up are Carole and Tuesday, Vivy: Fluorite Eyes Song, and Land of the Lustrous

Has no one watched Beck Mongolian Chop Squad? The anime is about a commercially successful rock band. The producers had to license a solid soundtrack of songs for scenes of the characters performing. The soundtrack is one banger after another.

Yet, somehow FLCL still beats it.

Made in Abyss has a phenomenal soundtrack that takes you along their adventure.

Hanezeve Caradhina is my personal favourite.

I've found myself listening many times to the whole soundtracks of One-Punch Man and Spy x Family.

• cowboy bebop
• samurai champloo
• berserk
• hellsing
• vampire hunter d
• Dragonball GT (though it pains me to acknowledge that series in any capacity)
• outlaw star

take your pick.

The original Hellsing anime. I love its mix of jazzy, blues rock guitar and funk.

Super tough I would say for me it is between. Cowboy bebop and initial d

Given that I just let a shipping company extort me just so that I could have the complete ARIA vinyl collection I'm gonna have to say ARIA for me 😅.

Gintama is also a great choice! It such a diverse anime and the OST reflects that. I love listening to it as well! Picking a favorite is hard but I think it is 昔の友人が変わらずにいるというのも悪くないものだな… It's very OG and has such an wholesome bittersweet sensation to it. Gintama has a lot of bittersweet and I love that. Life is bittersweet after all. My favorite character theme is probably Madao's theme.

Are we talking the OPs or like, the score?

For the former, Bungo Stray Dogs and Sword Art Online are easy picks, though they both only used a couple/few artists. Fullmetal Alchemist (both of them) had more variety and a good range. My Hero Academia, too.

Hard to pick a "best". Both Fullmetal Alchemist soundtracks were pretty good. I also really liked the .hack//sign soundtrack even though the show itself was boring af. Dan Da Dan and Gurren Lagann both had some inspired choices (fight the power!).

The Legend of the Galactic Heroes OVA is an obvious contender; it uses exclusively classical music for its soundtrack and it works really well. I can't hear Dvorak's 9th Symphony 4th Movement without thinking of people dying in space now.

Ghost in the Shell: Stand Alone Complex and SAG 2nd Gig which is....ok Yoko Kanno at this point this is cheating.

So instead I'll say Mobile Suit Gundam: Thunderbolt. That thing made Jazz terrifying.

So many good ones. Some already mentioned, but ones that weren't: Shield hero has two bangers. Fate stay has a few. Re: Creators. Dimension W. Gate's opening is really good and captures the feel of the show so well...Does Castlevania count? Cause that new bloody tears version is fire.

Macross Plus. Another Yoko Kanno masterpiece.

Someone already said Cowboy Bebop, so I'll say Kids on the Slope.

It's another series directed by Shinichiro Watanabe with Yoko Kanno doing a lot of the composition.

.hack//sign and gundam seed are both pretty iconic for me. Guilty Crown is full of bangers.

I enjoyed original Trigun’s guitar licks.

Panty & Stocking with Garterbelt, I frequently listen to the season 1 and 2 albums.

Air Gear story is okay but the music is great

One that was surprisingly good to me was K Project.

Blood Blockade Battlefront. So much variety!

Samurai Champloo