2 words is not enough.
For me, it's Joshua Tree, and the reason is quite simple. It was the album that opened my ears to music. I hadn't an appreciation for music until 1986. Then hearing the bass of With or Without You, the slow lead into Still Haven't Found, the organic intro to Where the Streets Have No Name.......it captured me. I spent alot of time going back and forth through the California desert during that time, and I found a connection of senses between my ears, my eyes and my soul. That was it. Over. End of game. I was a U2 fan for life at that point, and was able to go back, and live forward, to appreciate all the music they've given to the world.
Without a doubt, Joshua Tree.