The scene in the Extras Christmas special towards the end where Ricky Gervais' character talks about the idiocy of the celebrity culture he has gradually become a part of. It's finely written and it's his greatest moment of acting. It shows why he is one of the true greats working today.

Also, from another Christmas special, in The Royle Family where Caroline Aherne's character realises her waters have broken. It's a scene played simply and beautifully and her interaction with Ricky Tomlinson's character feels astoundingly pure.

The only one I can think of from a movie right now (and only because my sister brought it up recently) is the "mad as hell" scene from Network. If you haven't seen it then just fucking watch it. It's possibly even more poignant and relative today than it was when it was made.