Star Trek featured so many beautiful actresses, it's tough to choose one. Certainly Susan Oliver (in the original pilot) set the bar very high, in both white and green versions. :-). She gets my vote, because she was literally the prototypical Star Trek babe.
Shirley Bonne is a bit of a dark horse. She played Ruth in "Shore Leave" - who apparently featured prominently in Kirk's highlight reel. Shirley was one of the early "supermodels" back then, gracing over 400 magazine covers during the early '60s.
