I'm pretty certain it will be Rumours by Fleetwood Mac. It's the only album that I can honestly say I've listened to throughout my life, I heard it a lot in the car when I was a child, loved it and continued to listen to it regularly (at one point it was pretty much the only album I enjoyed listening to) and I still love playing it today. In fact I'm listening to my favourite song from the album, The Chain, right now.
Most of my other favourites have either been discovered more recently within the past 3-4 years or so, or I have gone for a couple of years or more without hearing them. There also albums that I have heard a lot when in the car with my family but which I am not really a fan of so don't have in my iTunes library and don't listen to them by choice.