What for you is the most iconic line of dialogue from Morrowind, Oblivion and Skyrim respectively? The one line you consider to be the most famous from each of those games? It can be an NPC-specific line, a general line that multiple NPCs say, an insult, a greeting, a PC line... pretty much anything.

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...

Daggerfall:
Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!Halt!
Morrowind:
We are watching you, scum!
Oblivion:
By Azura, by Azura, by Azura!
Skyrim:
yes, the arrow to the knee one... didn't know about the marriage joke, though

Personally haven't played Dagerfall, but I've seen YouTube clips of that. Pretty hilarious.