This will sound slightly unorthodox, but I recommend a 'crap' pair of Audio-Technica ath-m40x. Why recommend crap headphones? Because... even if you fork out £10,000 for a pair, they still won't be truly flat. However... you can get some very nifty software from SonarWorks called 'Reference 3'. They have analysed the frequency curves of a large number of headphones, providing an average calibration for each model (you can pay for a custom calibration but imo the average is fine).
The software is 99 euros, the headphones cost me around £75. You can demo it for free; check if they have an average calibration for your headphones, if they have, trust me CHECK THIS SOFTWARE OUT.
Everyone I've demo'd it to has been left slack-jawed at how it can turn a nasty crap pair of headphones into a beautiful, flat listening experience. And no, I don't work for Sonarworks, but I have bought the software and use it daily as second reference / nightly as a means to produce without pissing off the neighbours.
If you produce primarily on headphones, you NEED this software.