I have soft spots for particular scenes in certain shows. Three of my favourite Blackadder scenes are all in the same episode. The opening scene with Bob and her father, the doctor and the bit set in Putney.
'Please go on the game - it's a steady job and you'll be working from home.'
'That it be, that it be!'
'Yes, it is not that it be. You don't have to talk to me like that, I'm not a tourist.'
'You're just an old quack'
'I'd rather be a quack than a ducky.'
The whole bit where Blackadder is throwing a boozy party in one room, whilst trying to schmooze his super puritanical relatives in another.
The very last scene of Blackadder 4 personally showed me that comedy can make you cry as well as laugh and was one of the most powerful scenes ever, in a sitcom. A moment filled with masterful pathos and good though not insensitive humour (esp the "Great War 1914 to 1917" moment). Plus it was a superb way to bow out of a truly great series. For that reson alone, it's my fave.
Oh, and I like sad, if no one knew. 
Flash's entrance in the Blackadder II opener Bells. Was an average episode, then Flash entered and turned it into a classic. 'Melchy. You still worshipping God, last I heard you started worshipping me, ha ha ha ha ha ha'
Any part of 'Potato' from Blackadder II.
Blackadder's Christmas Carol was by far the funniest! But as far as the series went I like the second series when he's teaching Baldrick to count. Although the Macbeth one from the third series was pretty good too.
Whenever I think of 'Adder, the one scene that sums it up for me is the episode where Edmund is the executioner (I think) and he's talking to his err "assistants" and they tell him their name is "Ploppy". It cracks me up every time, if only just by Atkinson's pronunciation of things.
"Bob" was a good episode too.
As was Lord Flasheart, true scene stealer.
Socks are like sex....
The one with Tom Baker as Captain Rum shouting 'Rhubarb! Rhubarb!' on the boat.
The song from the first episode of Blackadder Goes Forth.
I think it's something like this: "Row, row, row your boat, gently down the stream, belts off, trousers down, isn't life a scream."
Absolute Magic!
I like it when Blackadder and George swap clothing and Stephen Fry beats George up for being a bad servant.
-Yes, M'lud
-(whack)I'm a Duke not a lord!!
First series.
Blackadder, when the castle is about to be attacked by an army of soldiers, and him being terrified.
Blackadder: Run to the hills, run to the hills...
Baldrick: My Lord they are coming from the hills.
Blackadder: Run from the hills, run from the hills...if you see the hills run away.
Quote: Clockface @ September 4 2009, 4:47 PM BSTThe one with Tom Baker as Captain Rum shouting 'Rhubarb! Rhubarb!' on the boat.
That's the one.
"You have a woman's hands!"
"Never ignore a poo poo."
