Oh, that's REALLY difficult!
I've been listening a lot to AC/DC recently (I love the new album).
My gut feeling is to suggest Back In Black and go forwards or backwards from there.
Another suggestion (if you like live albums) is get If You Want Blood with Bon Scott AND Live with Brian Johnson - you'll soon get a feel for which direction you want to jump.
Personally I love ALL the Bon Scott period. Although I bought Back in Black when it came out, I've only got into post-Bon AC/DC in the last few years - I listened to For Those About To Rock in the shop the day it came out and decided I didn't like the direction they were going in. Never bought another until...
I bought Black Ice when it came out, liked it, and I decided to get all the Brian Johnson albums and give em a good go... Depending on what you're after, some will be hit and miss, some won't. And for the one or two albums that leave you cold, you'll find they're someone else's favourites! (Btw, I really like For Those About To Rock now)
One thing, though - they all rock. For all they got lumped in with metal/etc, I personally think they are the most business-like and generally effective rock and roll band ever - they always do what it says on the tin.