Here are a few of my personal favourites. I hope you like.
Iman - Martia in Star Trek VI : The Undiscovered Country
Alfre Woodard - Lily in Star Trek : First Contact
Zoey Saldana - Lieutenant Nyota Uhura - Star Trek (Reboot)
And the one who started it all... the always elegant and eloquent Ms Nichelle Nichols ...
Lieutenant / Commander Nyota Uhura... Star Trek : The Original Series
(In the novels she becomes both a Captain and an Admiral!)
UPDATE - I didn't initially include Whoopi Goldberg because I am not too fond of the somewhat weird way she has gone these days. But she WAS very good as Guinan in Star Trek : The Next Generation.