The greatest single-playthrough game would be a fun category. I think my picks for that might be What Remains of Edith Finch, Gone Home, Broken Sword: Shadow of the Templars, or Grim Fandango. Fire Watch would probably get an honourable mention.

A "pinacle of a (mostly) defunct genre" category might be a good one too. I would argue that Command & Conquer: Red Alert 2 is the best isometric RTS games ever made.