Champloo is a great answer. I have been skimming through older shows I have watched to find clips to post to the community and when I was going through Champloo, I found myself just letting it run sometimes to just listen to it. I did eventually pick out a Champloo clip, so look forward to that sometime in the next week or so (it was hard to pick just one).
I think I'd have to nominate the Monogatari series. Each arc centers around a character, and that character's VA sings the op, not to mention most of them are incredibly catchy.
Macross Plus also has a phenomenal sound track, with some of Yoko Kanno's best work, IMO. It also ties in heavily with plot involving an AI pop superstar. It's really still pretty prescient today.
Some of the Yuki Kaijura soundtracks have been pretty outstanding as well. Both .hack//sign and Noir, for example.
As a Monogatari fan (🤮), I really adore the soundtrack. Not just the OP's and ED's but the background music too.
SutekimeppouKenka and Jinchiku playfully help carry long dialogue and deliver their "punchlines" (there's a lot more, I just can't find them on YouTube). Nisemono adds to Kaiki's mysterious and deceptive aura.
I like music in Haruhi Suzumiya. But if we're talking about The best soundtrack - it is hands-down Paprika.
Just listen to this one song and tell me with a straight face it's not the best thing you ever heard: https://www.youtube.com/watch?v=APIKVLw1tT0
There are some great answers in this thread already. Personally, the OST I have been listening to the most recently is probably the Kessoku Band songs from Bocchi the Rock. The album they released from that show is in regular rotation for me and has been since the show aired. Just some great songs.