There isn't just one. And there are some great ones here, most notably the B5 Delenn speech defending B5/Sheriden, the cliffhanger of TNG's "Best of Both World's" and Mal's engine kick.
I would like to add a few -
* Star Trek: Next Gen - The finale "All Good Things..." which, for my money, until Angel was the best genre series finale show ever. The scene where we learn Q's true relationship with Picard at the very end and his role in letting the Captain save humanity was classic. Put a bowtie on a tremendous series which just happened to have a subpar last season. But that ep and that moment was a great payoff and the best thing Brannon Braga ever wrote.
* Angel -Again, a finale "Not Fade Away." There were two. The entire scene where Wes confronts the wizard and Illyria's vengence. And then the last stand in the alley:
Spike: Any terms of a plan?
Angel: We fight.
Spike: Bit more specific?
Angel: Well, personally? I kinda want to slay the dragon. Let's go to work.
* Deep Space Nine - "Sacrifice of Angels" when the Federation races again the clock to retake DS9 and stop Dominion forces from coming through the wormhole which is having its mines deactivated. Yeah, not really a moment but the Defiant punching through the line (with help) is an image burned into my brain.
* Buffy - When she sends Angel to hell in order to save the world just after he gets his soul restored. Season 2 finale. Crushing scene. Out of all the Buffy moments, this is easily the most memorable. The show stepped up from really good to great at that moment.
* Babylon 5 - "Point of No Return" Sheriden's speech declairing B5's secession from the Earth Alliance. Trumped maybe only by the aforementioned Delenn speech.
* Firefly - "War Stories" There are two. You know what they are, but juuuust in case:
Book: Oh... my.
Kaylee: Well, gosh, I knew she took female clients but... They look so glamourous together.
Jayne: [after a beat] ... I'll be in my bunk.
and
Just after Serenity's crew blasts their way into Niska's spacestation, they get to where Mal is struggling with Niska's bodyguard on a ledge, Jayne starts to aim at the bodyguard and:
Zoe: Jayne. This is something the captain has to do for himself.
Mal: No. No, it's not!
Zoe: Oh! [guns a-blazing]