The "best" headphone sound is personal — it depends on your specific headphones, your hearing, and what you're listening to.
Rather than following a generic recipe, use the Bypass switch to compare your adjusted sound against flat as you go, and trust whichever version you actually prefer. A curve that sounds great on one pair of headphones can sound wrong on another, so this is worth doing per-headphone.
Ears adapt to whatever sound they hear regularly, so a curve you loved a month ago might feel slightly off today. It's worth occasionally comparing back to flat to make sure your settings still reflect a genuine preference rather than habit.