The Samurai Champloo soundtrack, easily.

Looking through the soundtracks I've bought , I have to confess, I'm a Yoko Kanno fanboy.

Cowboy Beebop - I admit to being one of those people who fires up Tank! when they start their car, although I alternate it with a few other songs. Piano Black, Mushroom Hunting, and of course Blue stand out.
Turn A Gundam - Imported all 3 soundtrack albums the Movie OST and the Concert CD.
Ghost In The Shell: SAC & SSS

Non-Y.K. soundtracks that could nearly pass themselves off as such (a high compliment):
Samurai Champloo: Still holds up well, and I pretty much fall asieep to "Thank you" every night. Damn that show needs a sequel!
Last Exile - Really fits the anime well. Cloud Age Symphony is some good driving music if you need to make up time.

Others:
Eureka Seven - A show largely comprised of unlikable people doing questionable things for questionable reasons, but good music.
Gundam Seed & Seed Destiny: Better soundtracks than shows.
Tenchi Muyo!

Stuff I've got digitally because I either never found the CD version or one with all the songs I want doesn't exist for sale (or is out of print):
Bleach - Like a good number of the OP & ED songs
Gundam 00 - Ash Like Snow is still on a few playliststs
Naruto & Naruto Shippuden - Who doesn't like Hero's Comeback?
After War Gundam X
Iria: Zeiram the Animation - Honestly, just for "At the end of the melting dream"
Tenchi Universe
MS Igloo - I hope this gets a re-release on Blu Ray in the U.S.
Inuyasha - Another one where I like the music more than the show.
Macross Pluss - my favorite version of Voices is the Cello instrumental version.

Random songs from:
Zeta Gundam - Still pissed they never got the rights to put the correct openings with the US DVD & BR releases. Stupid Neil Sedaka.
ZZ Gundam
Tenchi Muyo! GXP - the OP & ED tracks are damned good fun.

the one I best remember is "What's up people" by Maximum the Hormone in the beginning of Death Note; but I can't say with certainty it's my favorite

Penguin Research - Button (ReLife OP)

Nuff said.

https://youtu.be/OqKD5WOivpQ

of course "Tank!" from Cowboy Bebop is a classic, but I would also note "Hello, shooting star" from Assassination Classroom, mainly because it sounded familiar to Radiohead's "Fake Plastic Trees." I know, it doesn't seem like it, but the acoustic guitar lines in the song instantly reminded me of the start of the song for some strange reason. I still fucking love it regardless of that.

Mainly just mentioning some others have mentioned....

Death Note and Attack on Titan, openings and endings and all the music, really

Cowboy Bebop, though not the opening for me so much as some of the music from the series: the songs in the Vicious episodes, "Real Folk Blues", especially, and the song at the end of the series, etc.

some of the music in Space Dandy including the opening and ending theme

Samurai Champloo opening and ending theme

Eureka Seven has some good music, including openings and endings (never watched all the episodes though, as I found the series itself a little boring)

Fullmetal Alchemist and Fullmetal Alchemist Brotherhood have good music/themes
the opening to Serial Experiments Lain is a great song (couldn't finish that one either though)

Flowers of Evil, good opening and ending

And... I'll stop there

Daiabare! Zoro x Franky One Piece

https://www.youtube.com/watch?v=9ePWlO-9oSA

My favorite OST is Supernatural Haste from Haikyuu!!

https://youtu.be/0NRoi3xQMRQ

Re:Zero OP or Crossing Field/Ignite from SAO.

https://www.youtube.com/watch?v=ZWmIxjjIGls

Bleach 2nd opening, forgot what the song was called, lol

The Pillows music from FLCL is all really great
Rewrite by Asian Kung Fu Generation from Fullmetal Alchemist
A lot of the music from Durarara is interesting
the music from Yuri on Ice is entertaining and well written

Pita Ten opening is one of my favorites. Really happy song

https://www.youtube.com/watch?v=zrljKsLzL6c

https://www.youtube.com/watch?v=eKBvnIf_3Vk

"Rolling Star" by Yui is one of the Songs that absolutely cemented my Love for Japanese music.