Macross Frontier hands down! Music saves the world! There's not a single song I don't like. Megum & May'n have such great vocal talent. <3 <3 <3
Tsubasa Reservoir Chronicles is another with its music being composed by Yuki Kajiura. The music really reflects the epicness of the anime, and really comes to the forefront in scenes rather than being part of the background. There's this great mix of symphony, opera, and electronic synthesizer. The music is beautiful and haunting. My favorite song is Kaze no Machi he sung by the character Oruha (from CLAMP's Clover).
Sailor Moon--this anime's music really varies widely. There's catchy pop tunes to epic, fast paced battle themes to soft ballads. It has some of the best music. I remember back in my hardcore fangirl/otaku days, I downloaded ever single Sailor Moon song I could find, learned all the words (both Japanese and English songs), and sang them aloud all the time with my friends. The Japanese lyrics were actually quite easy to memorize, and I'm surprised at how much I still remember. Occasionally, I'll start humming a tune and start singing the lyrics of one of the songs. Two of my favorite Japanese songs, likely only known to die-hard fans are Sailor Team no Theme and Unmei wa Utsukushiku (Sailor Neptune's song). Rainy Day Man is probably my favorite dub song.
And lastly, K-ON! Fun, catchy pop tunes. Exactly what a moe schoolgirl pop/rock band is supposed to be. There's a reason why everyone's singing and covering "Don't Say Lazy" --it's awesome!