I reckon everyone has at least one. Some people have a few and some have many.
My first musical epiphany was "Even Flow" by Pearl Jam. I was going to say "Alive" but it was "Even Flow" that really woke me up and opened the door to many other top tunes. It's a great piece of grunge that has fine lyrics and a tune that is only just this side of uplifting and it made a lot of sense to me as an ugly and misunderstood teenager. Ten was also the first album I bought. The first single I bought was "Walk Like An Egyptian" by The Bangles (I think) but I only got that becasue I fancied Susanna Hoffs, the lead singer. No epiphany there I'm (not) sorry to say.
An interesting one for me is "Come Back Brighter" by Reef. With hindsight, it's not the best tune by a LONG way but it came out at exactly the right time in my life and caused a 'moment of clarity' that got me through some bad shit. It's often the timing and not the quality if you get my drift.
My most recent epiphany is "Harmonic Generator" by The Datsuns. First heard it at the Reading Festival this year and it's energy blew me away and it's still firmly entrenched in the stereo.
So, let me know what yours are.
[imitation is the sincerest form of flattery and this idea is mercilessly copied from a visitor.]