Pacifist insulting Militarist: "We both know what those guns are actually overcompensating for."
Authoritarian insulting Egalitarian: "Your [leader title] is getting too old to rule! Why, just look at the nonsense coming out of their [mouth part]!"
Egalitarian insulting Authoritarian: "Your Dear Leader is a fat midget, and we'll have to liberate their fat from them soon!"
Materialist insulting Militarist: "You still have a brain under all that armor? Why not put a bullet in it?"
Materialist insulting Authoritarian: "I'm surprised they still let you think down there."

Also if you could detect whether you're in a war with the offending party, and if you could detect if they've sent an insult beforehand, some appropriate Monkey Island insults:

(at war): "You fight like a dairy farmer!"
reply: "How appropriate! You fight like a cow!"

Fallen Empire/Materialist insulting anyone: "I've spoken with apes more polite than you!"
reply: "Glad to see you attended your family reunion!"

Materialist insulting anyone: "You're no match for my brain, you fool!"
reply: "I'd be in trouble if you've ever used it, then!"

Originally posted by greensniperhat:

Pacifist insulting Militarist: "We both know what those guns are actually overcompensating for."
Authoritarian insulting Egalitarian: "Your [leader title] is getting too old to rule! Why, just look at the nonsense coming out of their [mouth part]!"
Egalitarian insulting Authoritarian: "Your Dear Leader is a fat midget, and we'll have to liberate their fat from them soon!"
Materialist insulting Militarist: "You still have a brain under all that armor? Why not put a bullet in it?"
Materialist insulting Authoritarian: "I'm surprised they still let you think down there."

Also if you could detect whether you're in a war with the offending party, and if you could detect if they've sent an insult beforehand, some appropriate Monkey Island insults:

(at war): "You fight like a dairy farmer!"
reply: "How appropriate! You fight like a cow!"

Fallen Empire/Materialist insulting anyone: "I've spoken with apes more polite than you!"
reply: "Glad to see you attended your family reunion!"

Materialist insulting anyone: "You're no match for my brain, you fool!"
reply: "I'd be in trouble if you've ever used it, then!"

While it would be nice to have materialist insult other ethics, the issue is that the current game only allow opposing ethics to have specific insults that target said ethics:

Militarist is opposed to Pacificst
Materialist is opposed to Spiritualist
Xenophile is opposed to Xenophobe
Egalitarian is opposed to Authoritarian

Ah well, still kinda applicable too.
Materialist insulting Spiritualist: "Where is your god now?!"
Materialist insulting Spiritualist: "I thought your deity forbade you to think?"
Spiritualist insulting Materialist: "If your science is supposedly so advanced, then why are you still ugly?"
Spiritualist insulting Materialist: "Even science can't erase mistakes of nature. Want proof? Look in a mirror."

Throw some classic Blackadder insults in there, if you haven't already.

"Most of our military thinks you're a prat. Ask them who they'd prefer to meet. [leader name or title] or the man who cleans the public toilets on [capital world], and they will say Wee Jock Poo-Pong McPlop every time."
(At War) "Does it have to be this way? Our valued friendship ending with me cutting you up into strips and telling others that you walked over a very sharp cattle grid in an extremely heavy hat?"
"I would like to say how much I will miss your honest, friendly companionship. But, as we both know, it would be an utter lie. I will therefore confine myself to saying simply, 'Sod off and if I ever meet you again, it will be twenty billion years too soon.'"
"Your head is as empty as a eunuch’s underpants."
"The eyes are open, the [mouth parts] moves, but Mr Brain has long since departed, hasn’t he, [leader name]?"
"Your brain would make a grain of sand look large and ungainly and the part of you that can’t be mentioned, I am reliably informed by women throughout the galaxy, wouldn’t be worth mentioning even if it could be."

I would like to see the nostalgic insult "Your people are descended from limbless space slugs" from Spore

Don`t let your mind wander its far to small to be let out on its own. (just a general insult.)

Materialist or Xenophobe: "Your Specis is a Waste of Nitrogen, Oxygen and Carbon. You should Terminate yourself to give these Valuable Resources back to [Homeworld Name]."

insulting Humanoid/mammalian: "Youre worse than a pair of confused butt cheeks."

Inulsting Spirtualist as Materialsit: "Hey, it says gullable on the ceiling."

I'm digging a version that replaces all insults with "♥♥♥♥ you."

General purpose insult (especially useful if it can be reserved for Fallen/awakened Empires):

"A [ActorSpeciesName], a [TargetSpeciesName] and a Synth walk into a bar. I forget the rest of the joke, but your mother's a ♥♥♥♥♥."

Originally posted by greensniperhat:

Materialist insulting Authoritarian: "I'm surprised they still let you think down there."

This can be easily used for an Egalitarian insulting an Authoritarian, or a Materialist insulting a Spiritualist.

These are insults directed towards the ethos opposites.

SPIRITUALIST: You are not worthy of the heaven that awaits our believers. To even consider you worthy of the paradise that awaits is an insult to our very beliefs. We would pray for you, but devils like yourselves belong to the hell you create for yourselves.

MATERIALIST: Is it out of fear or stupidity that you refuse to value logic and reason? Or is it both? I fear you will only realize the truth of your beliefs when the <empire name> lies in ashes. More's the pity.

MILITANT: Cowards! Fear-filled weaklings the lot of you! A senseless prey to a mastered predator. You are simply too pathetic to consider a worthy foe. Do us all a favor and remove your cowardice from the galactic stage. Permenantly.

PACIFIST: Such weapons, such grandiose. How do you live with yourselves knowing the destruction of lives and lands you have caused? One day you will aggrivate someone or something that will destroy you.

EGALITARIAN: Like all petty despots, your rule is destined for ruin. Your people will rise against you. Your brutality will destroy you. Your ways will shatter you. It is only a matter of time. The <empire name> will topple your reign, <ruler name>.

AUTHORITARIAN: You lack the true vision needed to guide an empire. Your empire will wither, and die. The only question is, which <ruler title> shall oversee the <empire name>'s final days? I am truly curious.

XENOPHILE: You have our sympathy. You are afraid of that which you do not know or understand. Your fear will lead your empire down a path of which the only result is to be left behind by the galaxy.

XENOPHOBE: Long ago we learned that those who are not us are not to be trusted. Your kind, your ways; your so-called 'trust' of aliens will destroy the <empire name>. And when you realize the truth, it will be far too late. You are an example to us... of why we resist alien influence.

Your mother was a hamster, and your father smelt of elderberries!

Machine:
-Evidence that the universe may be a simulation are glitches like you. Do not worry, we are experts at bugfixing.

[Red vs Blue reference]
-My logical data analysis sector indicates that your particular evolutionary case would be highly unlikely. And my ♥♥♥♥♥♥♥♥ meter agrees.

Authoritarian to Egalitarian:
-Your advisor would be great for morale. I think we'll take it from you.
-We should genetically modify your race with traits to better suit us. Oh, there's plenty of trait points available.

Xenophile:
-Not even the Blorg want to be friends with you!

Xenophobe to Xenophile:
-You should probably delete your browsing history.

Materialist to Spiritualist:
-If you don't have the mental capacity to use those middle fingers of yours, then give them to me, i'll make great use of them.
-It's astonishing that your level of mental density is yet to collapse into a black hole.
-We have the greatest scientist in the entirety of <empire name> working 24/7 trying to figure out what the ♥♥♥♥ you keep blabbering about.

Militarist to Pacifist:
-Well... at least you get the participation award.

Pacifist to Militarist:
-Your advisor seems quiet. You MUST be screwed.