Somebody changes lanes without signalling, or runs a red light, or turns when pedestrians want to use the crosswalk… “Well, aren’t we a rare and special flower of importance! Must be nice being you.”
Picked up this one at work many years ago:
“You embryonic rat-brain precursor!”
“If you were Jesus’ mother (or father) he would have nailed himself to the cross.”
I’ve often found the OED quite helpful, but it barely fits in my back-pack.
As to this…
“Pardon Us for talking while You’re Interrupting…”
…if they still don’t get it…
Pivot right foot between them and the person you are talking to, facing Them.
Make eye contact.
“Excuse me… do I Know You…?”
…and if they still don’t get the hint…
“No. No, You Don’t. But That. Can. Change…”
Big Smile 
allow them time to walk away
*Turn and continue conversation with, “…now where we? Weren’t you just about to tell me about [and make damn sure you remember exactly what was being discussed]…”
I’ve been waiting for a real-life phone spammer to actually answer their phone so I could casually ask them “have you considered parthenogenesis?”
Garlic eating, stinking little yellow, greasy, fascist bastard… (Points for the source.)
Yes I actually use that sometimes. Under my breath, but still…
I remember sitting patiently while a student made up an ignorant and completely wrong answer.
I might have shaken my head in despair, saying:
“And you were the fastest sperm…”
Scrofulous spawn of a camel
10 PRINT “You’re family tree must be”;
20 PRINT “a cactus because everybody on”;
30 PRINT "it is a prick! ";
40 GOTO 10
I have a good friend who tells “your mom” insults to her own children!
Totally using this one…thanks! 
“Bastidge. Farging icehole.”
From the film Johnny Dangerously; one of the characters was a gangster who badly mispronounced English (particularly obscenities). Insulting but funny, and plausibly not actually naughty words.
“Your mother was a hamster, and your father smelt of elderberries!”
Absolutely must be delivered with an over the top french accent.
Ah faht in yoor zheneral direction!
(1) Only for irritating women: “You Communist tart.”
(2) For anyone: “Well pardon me all to hell!”
#2 is a John Wayne line, I think from the Shootist.
To anyone you’re trying to get a point across to who suffers Dunning-Kruger Effect.
Paraphrasing Time Bandits- “It must be wonderful for you to be so mercifully free of the ravages of intelligence”.
“If I needed any shit out of you I’d squeeze your head. Now shaddup and wander off, grownups are talking here.”
Go brush your tooth.
My dad’s has been for years: “Did your parents have any that lived?”
Mine (when I hear something dumb): “Thanks, Crayons.”
For people expressing shit I’m not interested in hearing, whom I don’t care if I piss off:
“If I wanted your opinion, I would have beaten it out of you!”
Stolen from a movie so long ago that I don’t remember what one.