TOS (so hard to choose) - Amok Time, its just so classic, so perfect, when ever I think TOS I think da da da da da da dun da da dun, and Kirk and Spock fighting to the death.
TNG - Time's Arrow, They finally weren't afraid to get a little strange with the whole Mark Twain thing, and I really like how Data had to face his own demise, it made him seem more human.
DS9 - (again so hard to choose) Through the Looking Glass - Sikso kick ass and taking names in the mirror universe, you can't get much better than that.
VOY - Endgame (Part II) - The worst thing to happen to Star Trek since TFF finally ended
ENT - Borderland - Brent Spiner
TNG - Time's Arrow, They finally weren't afraid to get a little strange with the whole Mark Twain thing, and I really like how Data had to face his own demise, it made him seem more human.
DS9 - (again so hard to choose) Through the Looking Glass - Sikso kick ass and taking names in the mirror universe, you can't get much better than that.
VOY - Endgame (Part II) - The worst thing to happen to Star Trek since TFF finally ended
ENT - Borderland - Brent Spiner