TOS: Starting it all, sowing the seed, novel concept at the time.
TNG: Story, characters, allegories, actual sci-fi; greatly expanding the lore/"verse" of Trek and removing much of the TOS campiness.
DS9: Visuals, pew-pew.
VOY: ... drawing a blank.
ENT: In a way, ENT revived the Trek utopia of TOS&TNG; whereas DS9 spent much time deconstructing it and VOY was too far removed to really convey it.
Not through ideals but by linking this far-away future to today: whereas the world Kirk, Picard, ZappBrannigan & Janeway live in seems alien and distant, we can directly relate to the "first real steps in space" theme of ENT; because we've already started the process.