TNG: The Crushers, Troi, and Riker.
DS9: Worf and Klingons as a whole, Kira, Jadzia. The only folks I really enjoyed were the Ferengi and Garak.
Voyager: Harry, I didn't mind the rest of the cast, but they disappeared in the shadows of the Doctor, Seven and Janeway.
TOS and Enterprise, I liked both casts without reservation.