It isn't just the guards!
The language can get pretty colorful - one that keeps coming up in my mind is "By Stendarr's sweaty sword arm!" (spoken by Merric after the final quest).
And of course, the frequent references by the Argonians to the river when speaking of the passage of time - I love this analogy!
Ooh, I haven't heard the "river" one!!!
Here's one that Jerric never gets tired of hearing:
"Dog's filth! You anger a powerful enemy."
Ha ha, yeah, that's right I did!

Molag Bal!!! I like that one too, lol.
In addition to the game's colorful language (typically very lore-friendly, too), I love how the voices are done. After spending time in the Fighter's Guild quest line with Aelif and Colors, and Wrothgar with all the Orsimer, I've come to enjoy the variety of female voices I've come across, as well as their speaking cadences and what not.
The different voices are much more noticeable to me than in Skyrim or Oblivion, where (*gasp*) it sounds like the same two or three voices over and over again (I know, I know, it IS !!).
Agreed!
The NPCs are fantastic in this game. I continue to be touched and impressed by them.
And amused. There's a female who hangs out near the Orsinium Wayshrine that I'll have to write down what she says the next time I'm there. She's giving her drunk male friend a hard time, justifiably it seems!
I have to stop and listen for this one, I have missed it somehow!
I've heard this in passing before, but I thought it was funny how Eolan got it frequently from the Nord under the Windhelm bridge while the other characters didn't even get it once when they participated in the event.

I heard that while passing nearby the Windhelm stables,

Is it just Khajiit, or is everyone in Windhelm drunk?
They are all Nords, what did you expect?

Not really an NPC per say but Khajiit found this little nugget from woodworking hireling Pacrooti to be hilarious:
"Today Pacrooti heard a voice say 'Khajiit! I need your help!' He turned around and saw a toad. The toad said 'I've been cursed by an evil mage. The curse will be broken if you kiss me. I will turn back into a beautiful maiden.' Pacrooti put the toad in his pocket. He can get a lot of gold for a talking frog."

You get new hirelings every so often. Pacrooti is the Khajiit who deals with crown crates though, so you can see him any time you want.
Haa-Rei just unlocked Gavin Gavonne, the provisioner hireling. He's one of our favourites:
"I hope these provisioner components reach you in a timely manner. I got turned about in a strange swamp and lost one boot to the mud. The nearest wayshrine was a short walk, but a very long hop."

Most any Nord: "I need another drink like I need a head in the hole."
I've heard that one too!
while crouching and trespassing in argonian homes sometimes they say " there is no moisture in what you are doing" i dont get it at all.

"I must apologize for the delay. I am traveling with a caravan of entertainers, primarily females. I am not used to girls, or their customs. They roll their eyes a lot, and seem to reply to everything by saying 'what?' I only recently realized they do that to give you the chance to change what you said." - Gavin Gavonne
ROFL !!!
"The skins of those you love shall fly as my banners!"
~ Molag Bal (sometimes when you close a dark anchor)

Ooh, I've never heard that one!!!
I just remembered the Khajiit noble on the Woodhearth docks in Greenshade. He's casually reading a book and when you talk to him he says "This one should buy a boat."
Good lord.

What, you're choking? Too bad you big baby! NOW FINISH THAT PIE!!

Was it a pie eating contest? Lol.
Random drunk Nord street-partying in Windhelm (cheerfully): "Who wants to see how hard I can punch?"
OMG, I haven't heard that one yet!!! That is hilarious!!! And you know...I can actually picture Jerric saying that too!

One of the random quotes from one of the drunk Nord women that I find funny: "Here's the thing-- I-I forget the thing."

"Little mortal, you amuse me so!"
Molag Bal, right? Lol.