My pleasure - I don't think you can go wrong with BIB as a choice - I am swayed by 'Rocker' on the first album, which my youngest daughter could sing at the top of her considerable voice from the age of six onwards!
There is something about a little girl singing
"Got lurex socks, blue suede shoes, V8 caaaar ... and tattoos!"
Back in Black is an amazing album without a doubt but for me it would have to be Highway to Hell or If you want Blood (Youve got it).
To be honest once you get one whichever you choose you will want more!