The MBP has a 16x10 display, so any resolution that is a multiple of that will give you the highest clarity. BUT when it comes to Overwatch, while you should select a 16x10 display resolution (like 1920x1080), you must select the 16x9 aspect ratio setting (it's a different setting). This will result in some black bars on the top/bottom of the screen while in game. If you select the 16x10 option, the game removes the black bars by effectively zooming in and you'll actually lose a small amount of displayed content on the left/right sides of your screen.
Higher resolutions resulted in FPS dropping lower than I'd like. And messing with the render scaler resulted in a less than consistent experience for me. But hey, it's easy to play around with the settings, so figure out what works for you.
TL;DR, For Overwatch only: Select a 16x10 resolution. Select 16x9 aspect ratio setting.
Finally I selected 16:10 and 1920x1200 (1680x1050 for better fps). I don't like to have the black bars for gaming.
I got like 60-70fps downgrading the settings. Render scale at 100%.