Well, twocows, you're talking about Macross Frontier's insert songs, and not Background Music, lol. Even if they -are- great, and they were used as background tunes sometimes, I don't think that's the point of this thread.
That said, I really like Death Note's background music. It's just... epic and climatic. The orchestras and corals give them an eerie feeling that fits the series perfectly. I have to say that Princess Tutu's also really awesome. The variations and arrangements of various famous music, from Beethoven to Chopin, mixed with exciting ballets really do stand out. That's also the kind of feeling I get from Simoun's BGM- classic tunes arranged to fit the setting, giving them a classic-yet-new feel.
And I'm sure many will mention Gurren Lagann's, so I won't even start on that.