Okay, this is probably going to be a controversial opinion. I think that someone coming in to Star Trek cold for the first time, and whether their approach is chronological, thematic, or simply for pure entertainment value -- they should start with Star Trek First Contact. It's an little in medias res, but it works.
It's an extremely entertaining fast-paced adventure that draws the viewer in. It literally tells you the mission statements of Star Trek as part of the dialogue, and it's literally the beginning of the story. What single installment would be better?