I think for me it would be “Cotton Fields”.
And I did like “Kokomo”- (hides)
Predictable answer, but Good Vibrations.
And, if you’d asked album instead of song: Pet Sounds.
Just pulling one off the cuff I’d say Sloop John B. If I sat down and examined the entire catalog I have a feeling I’d find something else that I prefer that’s not immediately coming to me.
,and a quick glance at the Greatest Hits album in my Zune I think God Only Knows might qualify.
Very difficult to choose just one. God only knows would be my answer if I really only had to choose one. Good vibrations is of course another masterpiece that’s close to heart, as is the 1966 recording of Surf’s up.
Of course Pet Sounds is the best Beach Boys album since it is also the best album of all time. Simple logic.
I would like to give attention to a few songs that I really really like but I just can’t say are my favoriite of all time in competition of the songs above: Do you wanna dance, When I grow up to be a man*, Don’t worry baby, This whole world, Tears in the morning.
- Beach Boys Today! is totally the Rubber Soul to the Pet Sounds’ Sergeant Pepper.
Don’t Worry Baby is probably my favorite, followed by God Only Knows. I like Kokomo, too. 
I always liked Wendy 'cause that’s my name.
I also like Don’t Worry Baby.
“Sloop John B” is my favorite. Runners-up would be “God Only Knows” and “Wouldn’t It Be Nice.”
I’ll go with Wouldn’t it Be Nice with Don’t Worry Baby as a close runner up.
And I liked Kokomo, too! KOKOMO WITH MUPPETS!
I’ll be even more cheesy and admit that I liked Be True to Your School!
I always liked In My Room.
And Kokomo was okay with me too.
I once read that Paul McCartney called “Don’t Worry Baby” the most beautiful song he’d ever heard. Who am I to argue with Sir Paul? “Don’t Worry Baby” it is!
Don’t Worry Baby is indeed beautiful; I’ll give a slight nod to The Warmth of the Sun, though.
First song that came to mind: “Help Me, Rhonda”.
- All Summer Long (Is there a true stereo mix of this available?)
- The Warmth of the Sun
- Good Vibrations
- Fun Fun Fun
- Wouldn’t It Be Nice
By the way, it doesn’t count because it’s not a Beach Boys song, but I still have to give props to their 1986 cover of California Dreamin’. Here you have the Beach Boys doing a cover of a song by the Mamas and the Papas, containing a splendid 12-string Rickenbacker solo by Roger McGuinn. To me, it’s a wistful tribute to much of what was great about mid-60’s Southern California rock in one tidy package, using elements of the Mamas and the Papas, the Byrds, and the Beach Boys. As far as I know, it’s only found on the 1986 compilation album Made in U.S.A.
Definitely Good Vibrations, though I also like Wouldn’t It Be Nice.
Shut Down. Catchy tune. harmony and background singing at its best, and excellent use of rhyme.
Oh, Sloop John B! Turn. It. UP!!! Everyone, sing!
followed by Help Me Rhonda.
I think that was “God Only Knows.”
I always like “I Get Around.” What a fun song.
Oops! You’re right! But I’m going to stick with “Don’t Worry Baby” as my fave!
I have a dozen favorites, and the answer I give may vary from day to day.
Right now, I’ll vote for "Caroline, No."