I’ve been listening to LOTS of Anime Soundtracks since the late 90s. Here is a short list of my top favs:
-The Vision of Escaflowne - Yoko Kanno & Hajime Mizoguchi
-Studio Ghibli movies - Joe Hisaishi 
-Saint Seiya - Seiji Yokoyama
What about you? Always happy to discover great music! 
Uhhhh that’s a good question!
Personally my top 3 favourites are:
I’ve got to add as an extra for me Madoka Magica has some great soundtracks!!
Going to have the check these out. I love the Ghost in the Shell: SAC soundtrack (and the new SAC_2045), and Aldnoah Zero.
I like opening theme from Digimon adventures season 1.
Sweet memories from the childhood 
Naruto - Yasuharu Takanashi music.
I like almost every anime song I listen to lol…
But… if the OST involves Kalafina or Hiroyuki Sawano I’ll probably be addicted to it for a while…
Honorable Mention to Anna Tsuchiya (Nana /RE movie) and L’arc~en~Ciel that were part of my childhood
Demon Slayer and Jujutsu Kaisen have amazing soundtracks. I still go back from time to time and listen to them
Easy creepy nuts both op and ed songs for Call of the night
The most memorable OST for me, and the ones which I still listen to today are:
The evangelion opening still gives me chills
SAME! 
OMG yes, that’s an old time favorite, such a throwback full of joy 
Absolute banger that song!!
This might give you a good laugh! Neon Genesis Evangelion: The Lost 4Kids Opening (Full) - YouTube
I totally agree. The best thing I’ve heard in anime
My favorite tracks from anime are the Evangelion OP, and the L’s theme from Death Note. For sure there are many more songs that just have sunk into my soul, such as the Pokemon OP or Bloody Stream (Jojo), but to the first two I listen on a regular basis
Cowboy Bebop, hands down
Death Note had such spectacular OPs and EDs, specially the second season, the creative gimnastics must have been over the roof! I dont know what they took, but I want some of that!
Yeah, the second OP is… uh… well… Someday my brother said it was the first anime OP he was skipping. However, the anime itself stays awesome as hell
If we go adding openings I have to add JoJo’s then, they did not miss a shot with any of the openings and I’ve got to say the pillar man soundtrack is an absolute vibe