The soundtrack for Monogatari, Gundam, Hell Girl, Fate/Zero, .hack//Sign, Scryed and Madoka Magica were all magnificent.
All of the character themes were damn awesome also (Kaiki for example)
Well pretty much anything by the composers are masterpieces. Naruto, SAO, InuYasha and a lot of other mediocre anime have some of the best damn soundtracks because they're produced by the same composers that composed these anime soundtracks.