Sharpest 35mm lens for D810

having discovered that my D810's resolution is adequate for large groups of people (approx 250 on staging) , I need to find a great 35mm lens to use  I am currently using the 24-70 but dont rate it much

will I get much better performance from a prime 35mm?

Was looking at the Sigma Art 35mm or Nikon 35mm   The Sigma sounds best according to DxO but sounds like their are a few samples with focus issues and I can't afford mistakes. Am I better off just getting the Nikon 35mm is it more reliable?

anyone with practical experience?

Mike