Cathay Pacific Asia Miles - Best Upper Deck Seat on 747 with OLD Business?

Is row 86 this best upper deck seat on the 747 with the OLD Business Class?

Yes. And 86A specifically is the best seat because it is set far enough back from the exit that you can use the lockers at the side as a foot rest. 86K is much closer to the door so you can't do this.

I also would choose 86A first. Side note, in my experience, sometimes the passenger behind you in 87A will also do the same and put their feet on the side cabinets. Depending on their height, this may result in you enjoying the sights (etc) of their toes\socks about 1-2 feet from you. What I do now is as soon as I know everyone's carry-ons are stored, I'll put a newspaper or magazine on top of the cabinet. If you are in 86A, your feet on the cabinet shouldn't bother anyone.