For me, Morrowind: "You n'wah!"
Oblivion: "Stop! You violated the law!"
Skyrim: "I used to be an adventurer like you, then I took an arrow in the knee."
The 'arrow in the knee' one is probably the most recognisable line of dialogue in a game of this generation, to be honest. Also, not sure if anyone else knew, but apparently taking an 'arrow in the knee' is a Norse slang phrase meaning 'getting married'. If Beth meant that as a sly little joke, it's pretty clever. Changes everything...