Of the 6 planes or faces of your teeth, the 2 sides most difficult to brush are...

1. 2021-09-28 at 1:31 PM UTC

   #1
   WellHung Black Hole
   The top outside face, and the bottom inside face. Folks, they require proficient rotational ability from your tendons and ligaments.
2. 2021-09-28 at 3:18 PM UTC

   #2
   

   Quick Mix Ready Dark Matter [jealously defalcate my upanishad]
   3 since you claimed 6 planes.
   
   under the gums and between the teeth (hence you use tape or floss string)