Industry standard, very flat. I recommend these especially if you’re just getting started: https://www.amazon.com/Sony-MDR7506-Professional-Diaphragm-Headphone/dp/B000AJIF4E/ref=mp_s_a_1_3?keywords=sony+monitor+7506+headphones&qid=1556547026&s=gateway&sr=8-3

These are bright and clear. They reveal a lot. The top end can be a bit harsh without Sonarworks so if you go this route you’d want to get sonar works (linked below): https://www.amazon.com/Beyerdynamic-880-PRO-Semi-Open-Headphones/dp/B079P9H1SH/ref=mp_s_a_1_3?crid=FG0M37DL8V5A&keywords=880+beyerdynamic&qid=1556547157&s=gateway&sprefix=880+beyer&sr=8-3

Software that works great with any headphones, especially the 880s. It makes the sound more flat and balanced: https://www.sonarworks.com/truefi

If you buy 250ohm headphones you’ll also want a small amp so it’s powered properly. This one is affordable and works great. You hook it into your Audio interface: https://www.amazon.com/Behringer-HA400-Ultra-Compact-4-Channel-Headphone/dp/B000KIPT30/ref=mp_s_a_1_3?keywords=4+channel+headphone+amp&qid=1556547301&s=gateway&sr=8-3

Also research and consider these (read good things but never tried them):

https://www.amazon.com/Sennheiser-Open-Back-Professional-Headphone/dp/B00004SY4H/ref=mp_s_a_1_29?keywords=sennheiser+studio&qid=1556547204&s=gateway&sr=8-29

All that being said if your budget is higher than those I’d research Beyerdynamics and Sennheiser top tier products.