My Dad.
I was about 5 or 6 years old and my Dad bought "Boy" and just played it over and over and over. All of those songs remind me of my childhood, especially "An Cat Dubh/Into The Heart", which was a particular favorite for Dad, and now for me.
It's funny, but the more popular they became, the less interested my Dad became. The last U2 album he played to death was "The Unforgettable Fire". God, I remember almost every day after school I'd come home and "A Sort of Homecoming", "4th of July" or "Promendade" would be wafting through the house. Dad bought "The Joshua Tree", but didn't really play it all that much, though I do remember "Bullet The Blue Sky" a lot -- probably had to do with the Jimi Hendrix influence.
I claimed U2 as my #1 with "Rattle & Hum" and became a fanatic, and have been one ever since. I've seen them 12 times and even took my Dad to one of the "Elevation" concerts. It was so great.
My Dad's love of U2 may have peaked with "The Unforgettable Fire" (though he does really like "Even Better Than The Real Thing", "Dirty Day" and "Beautiful Day"), but my love hasn't peaked yet, and that's an exciting thing.