Although I'm a huge fan of John McPhee, my candidate for the best piece to appear in The New Yorker is Lillian Ross' portrait of Ernest Hemingway. It had been recommended to me many times over the years and I finally looked it up in the library when I was in grad school. There have been many outstanding pieces - it one has to be at or near the top.
The question came up again recently. How do You like it Now, Gentleman? is outside their paywall. It has been years - decades - since I last read it, and it is still great.