Oh, where to start, where to stop!
This time for a change I will not mention the "my nosey is so tiny that I sometimes think the pixies gave it to me". I'll mention instead its sister-scene from Series II, where Rowan Atkinson had a chance to watch with a dead pan face how the two idiots are giggling over the turnip which look like a thingie.
Of course, the table scene with two Lord and Lady White Adders is absolute delight.
From Series III, Sense and Senility, the actors teaching the prince how to speak before public.
Blackadder: "Or, here are my genitals, please kick them".
From Series IV:
Apart from the very last scene obviously, I guess it would be the court scene in Corporal Punishment or some of the more romantic scenes in General Hospital.