Take your pick of 5 guys
I'll say Dragon, Sakazuki, Fujitora, Dr. Vegapunk, and Kuma.
Not including Imu or Luffy because they both seem emotional af
I'll take the WT100 result to look for characters:
I'll consider maturity in this context as equivalent to:
- Having a strong risk VS consequences awareness
- Having a lot of patience and not being too impulsive
- Having a good self awareness and the capacity to self-reflect deeply
- Having a great emotional intelligence
- Having a big empathy
- Having the capacity to express their emotions freely.
For the men:
- Jinbe
- Dragon -
Although he doesn't really allow himself to express big emotions for the sake of his cause
- Koby
- Joy boy I believe
- Rayleigh too
You didn't ask about women so I'm adding them in a second top:
- Robin on top
- Vivi
- Reiju
- Viola
- and maybe Stussy, I'm waiting to see more.