Oh geez that's tricky... My first instinct is to say Towa Tei's Sunny, the entire album. But the thing is, half of what I love about it is the nostalgia about how much I loved it the first time, and where I was when I listened to it. So maybe it'd be better to pick something you might've liked better under other circumstances? In that case... Hmmm... I'd say the audio drama Spines.