Jean Lorrah -- technically a three-off, I guess? One classic Trek and 2 Next Gen.
She had such a handle on Data and Tasha that I'd love to see what she'd do with characters from other shows, esp. Jadzia Dax.
For those of you who mentioned Vonda McIntyre, I sure hope you've read her Dreamsnake
, because if you haven't, you've been denying yourself a real treat. There's a reason it won all those awards...