In my eyes, there are two different ACDC eras. The one where Bon Scott was the singer, and the other being with Brian Johnson as lead singer. My favourite album, and it's tough to pick just one, is Highway to Hell. While I do prefer the Bon Scott era for ACDC, it doesn't mean I don't like everything from Back in Black onwards. They made that transition better than anyone could have ever guessed back then.
Definitely two eras
Bon Scott was more bluesy and Brian Johnson a little more shrill/heavy
I think a lot of it depends on who you were raised on.
My favorite album is "Back In Black" which is Brian Johnson, but my second favorite is "Highway To Hell" which is Bon Scott.
My 11 year old son and 9 year old daughter, who are hardcore ACDC fans both prefer Brian Johnson as that's more of what they heard when they started listening to ACDC (and they both watch the "Live At Donnington" DVD Religiously, so for them it's Brian Johnson all the way.
I don't really have much of a preference either. Both singers had distinct characteristics, but it isn't like one shines brighter than the other. AC/DC will always be one of those bands where the music survives generation after generation.