There are lots of songs in the BBs catalog where backing vocals are KILLER, during songs which might otherwise be considered subpar, or at least on songs which are not very well-known and are rarely talked about.
I hope that one day the ENTIRE catalog of BB songs, obscure or otherwise, will be released with instrumental-only, all vocals-only, and instrumental w/backing vocals-only (similar to Wouldn't it Be Nice from the Pet Sounds Sessions box)... not that this is likely to happen, but I can dream.
Examples:
Make it Big: I just happened to notice just how killer the backing vox are throughout Make it Big (I could go on about how much I dig this song in general, despite it being super cheesy in spots - the tag of the song alone is killer, with Mike doing what sounds to almost be a falsetto while trading off with Brian), but in general the backing vox on this song sound super like classic BB backing vox, with their voices sounding like they could have come straight out of the '60s.
Rock and Roll to the Rescue: some really neat counter melodies that Mike is doing during the group intro before Brian's main lead kicks in.
Endless Harmony: all the vox on this song, lead or otherwise, are rad...but especially the group backing vox.
Other examples? I'm sure there are many, but in particular I'm thinking of songs that have rad backing box, but the songs themselves are not necessarily considered top-shelf BB tunes (everyone and their mom knows that Kiss Me Baby has killer backgroud vox, for instance... I'm not looking for examples like that!

)