Day in the Life of a Tree...
Maybe not... I'd go for Time to Get Alone. There's some harmony but that could be taken by a brass/wind section.
My number one pick would definitely be A Day in the Life of a Tree. We need a version sung by BW.
My number two pick (though not a BB song, but obscure BW none the less) would be Darian's arrangement of Do You Have Any Regrets. Brian would sound great singing that!