The Beach Boys' "God Only Knows" only reached #39 in 1966. I personally dislike the song, but Oldies radio plays it all the time. I guess it "tested well". Go figure.
That's not the same thing. "God Only Knows"(Paul McCartney said it was the best song ever written.)is the flipside of "Wouldn't It Be Nice?", making it a double sided hit, in the same vein as "I Wanna Hold Your Hand/I Saw Her Standing There". It is also prominently featured in "Pet Sounds", which I have heard was the catalyst for the Beatles releasing "Sgt Pepper's Lonely Hearts Club Band" but I digress.