Mine...Mine was when I beat Pearl and was migrating some pokemon with my friend.
I had lost my one and only Sapphire about a year or two ago, losing my most precious pokemon, my Gardevior.
My friend shows me all the pokemon she's migrated and says "Hey...I think this is yours..."
And there she is! My Dizzara! The only survivor of my Sapphire, and happens to be my most loved pokemon, was accidentally left on her game when Dizzara was borrowed to help beat the Elite Four! I've never been very lucky, but that had to have been on of the luckiest moments of my life!
Today Dizzara got to level 100! She was about 70 when she come over to my Pearl.