Late-90s anime had tight budgets and fantastic music. Drawing is expensive and it still looked so-so in standard-def. Recording was massively cheaper - especially with computers - and stereo sounded great even on VHS. Audio was a solved problem during the awkward transition from traditional ink-and-film animation to high-definition digital wizardry.
So the original Hellsing (back when the manga had just started) had some occasional Quality™ scenes... but the OST is this jazzy shoegaze affair that's genuinely better than in Hellsing Ultimate.
Elfen Lied, which I wouldn't call bad but I wouldn't necessarily recommend, opens with this unreservedly fantastic choral piece over overt homages to Gustav Klimt.
The Legendary Hero is Dead! - I love the manga of this series, but the anime fell really flat. However, the anime did give us an incredible OP. (long version)
.hack//sign isn't as good as I remember it being as a kid, but the sense of strangeness and mystery is something missing from modern mmo anime. To their detriment, imo. When I eventually played an mmo for the first time, I was disappointed it wasn't "weird like .hack//sign".
It does still have top notch music, character design, art, and world building. It's just that characters get annoying and the plot is too slow-paced and repetitive. If you like other weird anime like Serial Experiments Lain or Ergo Proxy (which is also too slow-paced) then I'd say you should give it a shot.
Edit: As to your question, Ghost in the Shell SAC_2045 wasn't as good as original Stand Alone Complex (though I did finish it, so it wasn't bad, but it had a great second op. The ED for that season was also extra good.
Guilty Crown, as an anime it's nothing special, the openings by supercell and egoist are amazing. Not to mention the OST, which was composed by Hiroyuki Sawano, was absolutely amazing.