Some of the lightest, I think, would have to be "Take Me Out to the Holosuite" and "The Trouble With Tribbles."
Darkest...depends on what sense. If it's morally dark, I'd say "In the Pale Moonlight."
If we're going to go in the tearjerker sense...oh man, "Ties of Blood and Water." I bawled like a baby at the end.