@PieandBeer already posted it but as far as anime soundtracks go, FLCL's has yet to be topped in my book. No anime I've ever seen has as definitive a soundtrack as FLCL does when it comes to capturing and expanding on the anime's mood/atmosphere.
I've spent a lot of time listening to the various .hack soundtracks. (//SIGN is especially good.) I'm not sure if the anime has aged well, but the music is still pretty great!
Erased had a lot of really good tracks in it It's probably one of the few shows that I've hummed along with while watching it. I also really liked that many of the background sings felt very derivative of the intro and outro and all really meshed well together. I liked a few of the songs from Yuri On Ice!!! Especially the opening, but I don't know I'd that I'd call it my favorite per say. Durarara!! Especially the first season had a really good sound track too, and I absolutely loved its intro too. I'd probably Durarara!! Would be a close second. Maybe followed by Yuri On Ice!!
I have to agree with @CrackTheSky about the FLCL ost. Especially the first one: Addict. Not only is the music by 'the Pillows' excellent as itself, they also used it extremely well in the anime. Also anything by Yoko Kanno ranks very high on my list. Earth Girl Arjuna might be my favourite out of those.