Here's another one that is impossible to answer quickly.
However, unlike most of my friends, I have the advantage here. In that I do not watch enough shows to get a true headache.
Best scenes ever are scenes that garner a reaction from me. Doesn't matter if it's laughter or tears, they have to get a really strong reaction. And the best scenes normally are the ones that have me bawling. After giving this an extremely large amount of thought, I came up with two that I couldn't choose between. So I figured, okay, I'll do both.
Then I went looking for screencaps, and got more ideas. CURSES!
So. Best Scene Ever from Angel (2), Buffy (2), Firefly, and Dollhouse.
Angel, Season 1: I Will Remember You: "I'll never forget!"
Oh. my. god. Just looking at the images had me tearing up. The sorrow and agony during this final scene with Buffy and Angel is so palpable that if you don't feel something, your heart is even blacker than mine. Whether you support the 'ship or not, you cannot deny how absolutely moving this scene was. Stellar acting by both Sarah and David. You feel this scene in every fiber of your being, no matter how many times you watch it.
Angel, Season 1: Five by Five: "I'm bad!"
This was an unexpected end to the episode, and I think it pretty much knocked all the unspoiled fans on their asses. After such a hardcore episode, and an incredible fight scene, this was so powerful. They break through the window, and Faith is giving it her all and Angel's deflecting the blows. Faith gets more and more desperate, and then she breaks. Eliza Dushku was asked what her favorite "I'm a good actor" moment has been in her career, and she quoted this. I really agree. I've heard people say Eliza isn't a good actress. I DARE you to watch that scene and tell me she's not incredible. We never saw Faith like this before, or again. Massively a Best Scene nominee.
Buffy, Season 3: Helpless - "You stuck a needle in me! You POISONED me!"
This was...possibly the most intense scene in Buffy. There were so many hardcore scenes in Buffy in the seven years it was on air, but this one hurts the most. The trust between Buffy and Giles was so strong. Despite my 'shippy inclinations, he was like a father to her. And this betrayal hurt the fans like it had been done to them. And the acting between Sarah and Tony is...if you compare this scene to the "I don't wanna die" scene in Prophecy Girl, you can actually see how the two of them grew into their roles. This is pitch perfect in the most horrendously painful way possible. Just looking at the images of Buffy sobbing, with her hands in her hair, brings tears to my eyes. And I hear her deliver that line and it sends pain through my heart.
Buffy Season 7: Dirty Girls - "So you're the one who sees things."
This is a Best Scene Ever for different reasons than the others. I waited SEVEN SEASONS to see Xander finally have to pay for SOMETHING. ANYFUCKINGTHING. For seven seasons, he did not pay for any of his stupidity. His horrible treatment of Willow when she pined for him for so long. His going after her when she finally found love with Oz. His lying about remembering the Hyena possession. The infamous "Kick His Ass". His leaving Anya AT THE ALTAR. I. could. keep. going.
This moment right here? Was my payback. And I have no guilt about finding it one of the most satisfying moments of the series.
Also, Spike runs in and shoves Caleb off, which feeds my hidden Spike/Xander love.
Season 1, "Man on the Street: "I have a message for you, from inside the Dollhouse."
There are SO many great scenes from this show, but this was the original. I remember sitting on the couch in New York, my knitting forgotten in my lap, my jaw somewhere around my belly button. I was so into this scene already, the fight between Echo and Ballard so intense and hardcore, and then the flip got switched, and I was gobsmacked. This was the turning point for the show. Anyone who had stopped watching on the first few episodes needed to come back NOW, because shit just got real. From this point on, everything became 100 times better. Such an incredible scene. Not to mention that while this was happening, we also discovered Mellie was a sleeper. So. fucking. amazing.
"Heart of Gold" - Nandi's death
This episode is never talked about on the list of favorite Firefly episodes, and I do not understand why. I watched the entire series in one sitting, days after Hurricane Katrina hit, while in Florida. And I loved every single thing about this episode, most especially Mal's relationship with Nandi. This was not something we'd seen from Mal before. From the beginning, the two of them had such intense chemistry. And I already loved Melinda Clarke from her role as Lady Heather in CSI, so the fact that these two had one of the hottest sex scenes on any of Whedonverse show meant a hell of a lot to me. And then they killed her. And Inara sobbing over her body, and Mal's face just...that scene stuck with me. And it really hasn't stopped being (for me) the most powerful scene on the show, even more than anything from The Message.
And I could probably list 100 more if I thought about it too much.