Here's a few of mines, some might be old. Do post yours!

Guard: Let me guess, someone stole your sweetroll.
Me: Mmmhh-mm (can't say more with my mouth full of the sweetroll I just snatched from your pocket).

Benor (after challenging me to a brawl and losing sorely): You've been a good friend to me. That means something.
Me: Errmm..you may wanna check out Haelga in Riften instead.

Guard: My cousin's out there fighting dragons, and what do I get? Guard post!
Me: (points at the sky where the roar of an approaching dragon comes from).

Seamstress of Radiant Reinments: If you're heading to Blue Palace, you might want to rethink your outfit.
Me: (wearing Radian Reinments', heading to Blue Palace indeed) Good idea. Honesty appreciated.

Farengar: Come to Dragonsreach to discuss the ongoing hostilities like the rest of the "great warriors"?
Me: No, I actually came to buy off all of your soul gems and make you rich, but I just changed my mind.

Farengar: You know, if you got the aptitude, you should join Mages College in Winterhold.
Me: (facepalm)

Nazeem: Do you go to the Clouds district often? Of course, not.
Me: Of course, not. We all know too well that it's impossible to buy a decent swetroll in this skeeverhole of a city, so I hang out at Blue Palace in Solitude instead.

annoying repetitive NPC: I used to be an adventurer like you...
me: *shotgun to the face*

problem solved.

Punch in face with axe is the response I would like to give most often. Best of all it's possible to do.

"IGNORE ME!"

"Do you have any Grey Poupon?"

Dirge: I don't care if you are best friends with the guildmaster, start ...
Me: I AM THE GUILDMASTER!!! And you better start growing gills because you are ending up face down in the gutter.

Lydia blocks the exit
"MOVE YOU *****ING SLU**** SON OF A"

Gaurd: I used to be an adventurer like you then I took a arrow to the knee
Me: If I hear that from your mouth one more [censored] time I will feed you to the [censored] dragons!

Some people don't think I'm smart.


Those people get fisted!

OP, trust me when I say "You Already CAN talk back to NCP's.
You just have to live with the consequences of said choice.

Olfric Battle-Born - "What? Is he rumagging for rubies and rubbish? Get a job!"
Me - [Rough Translation]"Force... Balance, Push!"

Talos the Mighty! Talos the unerring! Talos the unassailable! To you we give Praise! Aye, love! Love! Even as man, great Talos cherished us. For he saw in us, in each of us, the future of Skyrim! The future of Tamriel! And there it is, friends! The ugly truth! We are the children of man! Talos is the true god of man!

Me: Odahviing, come shut this (censored) up!

Heheh..... I use Lydia as my companion too, and I can't count the number of times she's blocked a doorway
in front of me on a dungeon run, and I nearly scream through my t.v. yelling at her to get out of my damned way, the stupid
mare!!:-/

Well I've used more harsher words than that, but out of politeness, I won't say them on a public games forum.

I would tell her to "get the f*** out of my way or you'll be on all fours again trying to recover!"

Guard starts talking...

Run past him really fast!

But usually it`s `yea, sure.`

`Ok`

`why are you repeating yourself you said exactly that last time.`

`Morning`.

`Please continue, my good sir.!`

Run fast past them again, leaving them talking to themselves.

" I could be off....'
"What hunting dragons? I'll do your shift, Garda, not a bother. No one will notice with that helmet. You work away. Here, let me mark Shearpoint on your map."

Bandit surrenders, pleads for life, gets up and attacks again :
"Fus ro dah, that's Draconic for 'should have stayed down'."

Guard (for the first time): Wait. I know you!
Me (flattered): Finally! Autograph from Dovahkiin? Sure, no problem!

***

Guard (for the 2nd, third, n-th time): Wait. I know you!
Me (walks past real swiftly): No, you don't.

"My favourite drinking buddy"

"Look, I was new round here, I thought I was being nice, I got you that argonian stuff, leave me alone!"