PLUS ONE BILLION INDIE POINTS aside, I'd be interested to hear some examples.

Is it the first track that lead you to get deeper into band's work, but remained special for that reason (on a compilation, for example)? Did a band go commercial but keep their best material back to avoid alienating people? Have you not bothered to check out the rest of their output? Are you simply a die-hard and know their releases back-to-front?