I think it could be a good idea when everybody post his/her favorite episode of each Star Trek series. So the dev from STO can see what kind of Star Trek the people from this forum like. Maybe they can integrate some parts in STO.
TOS: Mirror, Mirror (2.04) - Kirk meet the bad Federation from the mirror Universe
TNG: Tapestry (6.15) - Q give Picard a second life
DS9: Little Green Man (4.07) - Quark, Nog and Rom are the Roswell Aliens
VOY: One Small Step (6.8) - VOY discover the relicts of the first human Mars mission
ENT: Dear Doctor (1.13) - Morality and philosophy in ENT (actually a typical TNG episode)
TNG - Toss up between "Yesterday's Enterprise" and "Parallels"
DS9 - I unfortunatly did not have cable during most of DS9, but of the ones I saw, the one dealing with the "Mirror Universe" was probably my favorite
VOY - see above, but of the ones I saw I think the "Photons be Free" episode was my favorite (I really don't know if that is the name of the episode, but I think you all probably know the one )
(Edit-Come to think of it, Jeri Ryan's proud posterior kept me watching the end of that series once I got cable.
Also, I swear her chest was all Borg Implants.......... )
Enterprise - I watched about 1 and 1/2 seasons of this series, but just couldn't get into it. What really kept me watching it was Jolene Blalock's proud posterior in that uniform.....hehe
My favorite movie was probably "The Wrath of Kahn"
TOS: agreed with OP, seeing it started the AU I actually liked (ENT's was just... weird)
TNG: Suspicions (6x22) and Lower Decks (7x15)
DS9: way too many - Way of the Warrior (4x01/02), Sons of Mogh (4x15), The Magnificent Ferengi (6x10), Far Beyond the Stars (6x13), Treachery, Faith and the Great River (7x06), The Siege of AR-558 (7x08)
VOY: Basics (2x26/3x01) mainly for the Suder sub plot; Unimatrix Zero (6x26/7x01)
ENT: The Andorian Incident (1x15)
I have so many favorites it's hard to narrow it. The two that first come to mind though are 2 TNG episodes. Parallels and Picards life in the civilization that gives him that flute he loves. Great episodes.
TOS: It's a toss up between The World is Hollow and I Have Touched the Sky and Spectre of the Gun
TNG: I would have to say Ensigns of Command.
DS9: Since... I haven't seen an episode of DS9 since I was about five or six years old when it first came on, I would have to say Trials and Tribble-ations since I saw that on my TOS set and I loved it.
VOY: Definitely my favorite but really close favorite between TOS/TNG. I would say this is the two parter Future's End. Absolutely love this episode.
ENT: Actually... never got to see it. It came on UPN and we never got that channel, it only came on cable where I lived and we didn't have it so I've never seen an episode of it.
Ive only watched TNG, but i loved "Darmok" for its intresting message and how the different approches of each captain was to their race and "Parallels" just messed with my mind a bit but it was very damn cool oh and gotta love bearded riker.
TOS: Whilst never being a huge TOS fan, The City on the edge of forever would be my favourite.
TNG: Family - the messages conveyed in that are pretty powerful imo.
DS9: Way of the Warrior (complete - not one specific of the episodes).
Voyager: Probably Q2. TNG and DS9 were my favourites ST wise, but there werestill Voyager episodes i enjoyed, and of those Q2 would be at the top of the list.
(Un?)fortunately, never saw any Enterprise episodes, it wasn't very popular down here.