Which open-back headphones are best for mixing and mastering?

The Sennheiser HD 600 is the name that comes up most consistently among reviewers as a neutral reference, with the HD 560S and Audio-Technica ATH-R50X cited as lower-cost alternatives with accurate tuning. Note that some reviewers, including audioreviews.org, push back on the HD 600's bass extension and soundstage, so the 'reference' label is about tonal neutrality rather than perfection.