I love the Smiley Smile 'Wind Chimes' - there's a harpsichord (?) solo piece which is just stunning, plus lots of tremendous group harmonising, particularly when they're working on the fade.
That is a smashing, and highly illuminating session. Smiley SOT is great
I think my favourites are the Pet Sounds album sessions. I got the Yellow Dog boot years ago, which was my first introduction to these sessions. Very bad quality, but magical. Also love any sessions from the '65 period. Can't get enough.
The OP touched on how lucky we are to have such a wealth of sessions. The Beach Boys themselves started the ball rolling with "My Favourite Recording Sessions" (what other band ever did that?) I know it's filler, but I used to find that utterly fascinating back before I knew what a bootleg even was.
Then of course we get "Stack' o Tracks". So the Boys themselves seemed to be aware of how interesting this sort of thing is.