Episode 1 of Series 6. This had it all: continuity, impersonation, internal politics, a foreboding cutaway, espionage within espionage (evoking T-Bag and her 'spying saucer') and a sinister outcome, even if it was easily remedied. I can't dwell on this scene, though, as I end up with the music of Rupert Holmes stuck in my head.
Episode 2 of Series 8. Starting with the game of Monopoly and continuing with one hilarious line after another.
Episode 5 of Series 8. The Bimboid. I'd like to imagine that they had fun filming this scene, and it does sound like Mark Knight is holding back laughter at one point.
Episode 10 of Series 8. The Dungeon Master defenestrates what little dignity he has left by openly using a spyglass on the Opposition. His fitting reward is the shortest, sharpest spyglass scene ever, and for Lord Fear to have the last word of the series.