I've flown the 787-8 several times in CW and on my last trip discovered that the middle seats (E) seem to have the most room width-wise when laying flat. Granted it was a near empty cabin so I didn't face issues with seat mates. But with both partitions raised, the E seat had more room than the A and K based on my laying in both.

I'm flying the new 787-9 and am going to select 7E (rear-facing, middle in the mini cabin with unrestricted aisle access. Will update this post after my flight hopefully with a couple of pics.