TOS is probably the most dated in its look but the quality of the scripts means it still holds up. The most dated element of the production for me is the music. Especially the music that plays for the 7 second awkward pause after a "joke" has been made.
The most futuristic Trek is The Motion Picture. Though that loses points for McCoy's disco outfit.
I don't think the music is 'dated', I just know every damn cue. I can play them in my head. I know exactly what humorous cue you're talking about. (Actually I'm thinking of the whimsical, puzzled cue as featured in Tomorrow is Yesterday. "A son, I...have a son")