I'd say Justice, but what would also be good would be to follow it with a less commercial souding Black Album, then cut out the Load and ReLoad crap, redo St Anger WITHOUT letting Lars and Kirk write lyrics (honestly, a 5 year old could do better...), tune up the guitars and throw some guitar solos in (I think St Anger was actually quite good as a one-off, which is what it was always intended to be, a sort of experimental garage-rock sort of thing, just a few things weren't done right...), then followed by Death Magnetic (which i love ALMOST as much as the old stuff.
I've got very high expectations for the next album.