Well, it depends on what you mean by "impact". For me, the album that just blew me away the first time I heard it was Leftoverture. There weren't any emotional ties or stories in any of the songs that hit home or anything like that. It was the musicianship and the brilliant song writing which incorporated and blended the instruments so incredibly well - and they even had a violin player!! A fucking violin player!!
I was just starting to break out of the "classical music only" kind of piano lessons I had endured for so many years and was wanting to learn to play everything I heard that I liked on the radio. So, when I heard that album with every song having such a commanding presence of keyboards and how BIG the sound was, I decided there and then that playing keyboards in a band was what I wanted to do - and I did.
As for a single song that just grabbed hold of me and wouldn't let go, that would be "Don't Let Him Steal Your Heart Away" by Phil Collins. His album, Hello, I Must Be Going, had come in the mail from Columbia House and really only knew 2 or 3 of the songs - which was fairly common back in the day when radio was the only real exposure anyone got to artists' music which would include a couple of songs (maybe) in their playlists leaving the remainder of an album to fade into obscurity. One night I put the album on and went about my business. When it got to this song, it stopped me dead in my tracks. My girlfriend of 4 years, my first love, had just dumped me and I wasn't dealing with it well AT ALL! I was 17 at the time. Ya know how it seems like every sentimental song all of the sudden is speaking directly to you..? that was pretty much the case with everything I saw, heard, read, etc. Then this fucking song comes on and it was like Phil hear about my situation and said, "Steve, I'm gonna write a song that says everything you wish you could say to this girl and every word will be taken straight from your heart. Sound good...?"
http://www.youtube.com/watch?v=vjQbxyxog1o