For me, it was Tuvok. Not only did he show that not all Vulcans were carbon-copy clones of Spock, but he kind of cemented the idea that Vulcans suppressed their emotions because they were more dangerous or self-destructive than Human emotions.
Second, was the one-time only appearance of the Swarm. To me, they were what the Borg originally were--a mysterious, hostile race you really couldn't communicate with (much less reason with) that simply kept coming at you until they overwhelmed you with superior numbers. The Voyager barely got away from them the first time--but if the Swarm had the ability to adapt to the ship's technobabble solution, it might not have, IMO. Another confrontation would have required them to have been watered down, so it was probably right they only appeared once (I'm deliberately ignoring the few seen casually in the background watching the match in "Tsunkatske").
Second, was the one-time only appearance of the Swarm. To me, they were what the Borg originally were--a mysterious, hostile race you really couldn't communicate with (much less reason with) that simply kept coming at you until they overwhelmed you with superior numbers. The Voyager barely got away from them the first time--but if the Swarm had the ability to adapt to the ship's technobabble solution, it might not have, IMO. Another confrontation would have required them to have been watered down, so it was probably right they only appeared once (I'm deliberately ignoring the few seen casually in the background watching the match in "Tsunkatske").