I think "Little Girl I once knew" is an overlooked gem of a song.
I know the later albums are more respected but I am more likely to reach for Surfer girl, Shut Down Volume 2 and All Summer Long over Pet Sounds and Today even though I agree they are better albums. There is just something about their early stuff which I love.
My favourite Beach Boy songs always change from time to time but I think that fundamentally they are
1) Don't Worry Baby
2) I Get Around
3) The Little Girl I Once Knew
4) Wouldn't It Be Nice
5) Hawaii
I love most of their songs really and apart from the other better known songs which have been put forward I will give special mention to In the Parkin lot, Farmer's Daughter, Surfer Moon, Hushabye, We'll Run Away, Good To My Baby and Don't Hurt My Little Sister. All songs I listen to a few times each when I go on a Beach Boys session.