On Star Trek: Deep Space Nine, the idea of Jadzia Dax was interesting--an old, experienced creature who'd experienced many lifetimes (like incarnations), combined with a young person. But I felt the idea was badly executed--both the writing and acting.
Of course, for everyone who was a fan of Terry Farrell, the answer to my question is "Terry Farrell."
But for those of us who feel she didn't do so well in the role, another actor could have improved the character a lot.
So--who would have been a good choice for the role of a wise, experienced joint being in a young body?