As far as text's sharpness goes, it should look the best at 1440x900 on 15 inch (it's "half" of display's real 2880x1800 res.). At other resolutions, you should notice that the text looks a bit more blurry.
The downside is less space for programs on display.
The benefit, apart from the better looking text, you might get slightly better battery life. Because due to how it all works in macOS, more resources are needed to use 1680x1050 scaled res than 1440 x 900.
the integer scaling (1440x900 for 15" 1280x800 for 13") is technically the sharpest, and it is noticeable, but probably not to the extent that it outweighs having more room to juggle windows about in.
Right, native is generally "best", and just to emphasize this post: 2011/12 pre-retina 15" MBPs were 1440x900 standard, with BTO options for a 1680x1050 glossy and 1680x1050 anti-glare display. I had one the latter models, was a decent upgrade over the base display (and the AG was nice for outdoor use).