the rarest healthy big five personality type is probably:

+ high openness
+ high conscientiousness
+ high extraversion
+ high agreeableness
- low neuroticism

basically: creative, disciplined, social, kind, and chill. very rare, maybe 1% of humans.

why? 

it’s hard to be all those things at once. 

openness and conscientiousness usually pull in opposite directions. 

and people who are outgoing and assertive aren’t always the most agreeable or emotionally stable.

this profile shows up in balanced, high-functioning people -- charismatic leaders, wise creatives, or unusually well-adjusted founders.

sources: 

mccrae & costa (1999), digman (1997), thalmayer et al. (2011)