I agree w/ everything you said except I differ in value that I give to runs and RBI just because I believe that they are affected so much by opportunity. If I have the best OBA on the team, but bat 7th ahead of two .200 hitters, I'm not going to score many runs. If I bat last, I'm not going to drive in many runs.
My favorite single stat is OPS - which is the sum of your favorites - on-base PCT + slugging PCT.
I agree with the usefulness of OPS. I use OPS as the sort column for the team batting stats.