Dim, I agree Brian Wilson is a genius and Pet Sounds is a fantastic album. But I'm not sure if it's the greatest piece of music ever written. I'd recommend listening to Vivaldi's Four Season's -- Winter in particular. Besides that J.S. Bach's Goldberg Variations and his Ciaccona for violin and four voices is likewise brilliant.
Come to think of it, maybe I just have a preference for violin compositions.
I wish Brian Wilson and John Lennon could have got together and made some music -- can you even imagine how great it'd be? Lennon's 'Imagine' might be my favorite pop song.