Mine would be when J'Ghasta, the way he says it seems so genuine and scary:

and:

Emperor Uriel Septim:

Right up there with the best of them is, in my opinion:

CUTTER: "My Lord has a real edge I'd like to cut myself on."

Her voiced delivery of the line makes it that much better.

-Decrepit-

Anything involving the common mudcrab

A line stolen from Act 1, Scene 2, of Shakespeare's Julius Caesar.

"Stop right there criminal scum!"

Did you come up with that all by yourself or did you look it up?

Agreed, J'Ghasta is awesome.

http://bilder.hifi-forum.de/medium/581738/watch-out-we-got-a-badass-over-here-meme_122324.png

My first favorite quote:

"It's a little too rugged up here for me. Add a couple hairs to a Nord and you've got a bear."

by Volanaro from Bruma.

and a second one:

"I heard a rumour that you're an idiot, is there any truth to that?"

by Owyn the Blademaster.

I knew that sounded familiar and a little too timeless to have originated in a videogame. But if you are going to copy, there is no one better to copy from than the bard himself.

Personally I would say it's more accurate to call it a paraphrase, rather than a theft, since the meaning and several relevant words are different. Here is the original (a soothsayer in the crowd is about to warn Caesar of the Ides of March):

I hear a tongue, shriller than all the music,
Cry 'Caesar!' Speak; Caesar is turn'd to hear.

You'll note I delivered my "stolen" comment with a

I'd agree with you, except that in Emperor Uriel's case, the statement is really off the wall. It's all metaphor, and it doesn't make any sense in the game context. I think they just wanted to take advantage of their Shakespearean voice actor. I can't really blame them.

Everybody steals from the Bard, anyway.

Heh! Yep. Even Gene Roddenberry, creator of the original series, the sequel of which made Oblivion's Shakespearean actor famous, borrowed from the bard with episodes entitled All Our Yesterdays and By Any Other Name.

I saw a mudcrab the other day. Filthy creatures, I can't stand them.

IN B4 DA LOCK!

Without a doubt a line during the mages guild quest line:

Don't be a jerk.

That might be his favourite line of dialogue.

Never seen your kind before. What the hell are you?

Valen Dreth to a mod race character at the start of the tutorial.

J'skar is so awesome, I actually got worried when he said that.

This one was so funny.

"Poof. It is exactly like magic." - Ancotar

I like the way the guards say "If you run into trouble, come find me" when your on their good side. It's kind of reassuring to know they've got your back.

"I--It's you! The Hero of Kvatch! This is truly an honor!"

They just sound so sincerely shocked to be meeting you. Love it.

"I once dug a pit and filled it with clouds. Or was it clowns? They started to smell after while...yeah, they were clowns." -Sheogorath.

"You have my ear, citizen." -Every guard in Oblivion.

"STOP RIGHT THERE, CRIMINAL SCUM!" -Every guard in Oblivion.

"The young folk just don't appreciate a good stick these days." -Some elf in Crucible; I can't remember his name.

Anything Uriel Septim VII says, because he's Patrick Stewart. Favourite line of his is "These are the closing days of the Third Era," because it sounds like the opening to Star Trek: Next Gen.

I liked one of Mankar Camoran's lines, but I can't remember which one. But Terrence Stamp sounded awesome as well.

"Ha! You move like a pregnant cow!" -Bandits

"Why do Khajiit lick their butts? To get the taste of Khajiit cooking out of their mouths!" -Dar-Jee (I still love Khajiit; just ROFL when I hear this).

"No, you can't see me! You can't see me! He can't see me! No one can see me." -The crazy Khajiit at the portal to the Shivering Isles.

I also second the "I heard a rumor that you're an idiot" line.

EDIT: Oh! "By Azura, by Azura, by Azura! It's the Grand Champion! Standing here, next to me!" -The Adoring Fan

One of my other favourite quotes would be when Mythic Dawn Agents say:

The way they say it seems so devote and fearless.