Paprika has a great OST, nothing can beat Susumu Hirasawa.
Kamichu! has some good tunes too, but that's just me really liking medieval songs...

Jigoku Shoujo has good music too, just not always...
Death Note, nice tunes, I like Hideki Taniuchi's songs better than the work of Yoshisa Hirano tough Hirano has some great tunes as well, some even a bit Complication from Nine Inch Nails-like...
Elfen Lied, cello's...nice

As for OP:
Elfen Lied, tough the saints-version is better, Death Note's OP 2 What's Up People by Maximum the Hormone [probably the only one thinking that...], Higurashi no Naku Koro ni OP [Kai's OP sucks, except when played backwards] And Zombie-Loan's Ookami no Nodo by The Birthday [I probably am, yet again the lonely wolf in this choice, hmmm wolf, nice pun]