You're a skid mark on the underwear of humanity.
You absolute fuck nugget.
(my friend said this one) You twat on a stick in a field of idiots.
You're a total douche canoe.

You are a scumfuck bastard mental dickface that should be gunned down in the street like the degenerate you are
(look at that, banned from League again)

The best ones are when you know something about them personally and can remark on that. Something they can't ignore because it's true and they hate that it's true. They internalize it and never forget it and it destroys them. So it depends on the person! :)

You're such a boob pocket

You're a fuckle(nut)

1- Ye're an armless, boneless, chickenless egg
2- OH YOU FLIP
YOU FLIPPING FLIPPING FLIPPER
OH IT'S THE KNACKERS YARD FOR YOU, PAL!
3- you're the worst thing since sliced Hitler

That last ones definitely gonna be added to my repertoire of insults xD

You breathe like you've got a curd of snot sandwiched in your nostrils.

"YOU COULDN'T DRINK A BEER WITH INSTRUCTIONS ON THE BACK YOU BLOODY TAMPON FUCK BRAIN" -a drunk Irish man in a bar while I was in Ireland

As an Irishman I approve that insult

I know I'm funny

No you're not

SHUT UP ME

NO YOU'RE NOT FUNNY

You couldn't poor water out of a boot if the instructions where on the heel
My favourite

1. "Double bugger sassafras horseradish balderdash" (for use in front of kid siblings-- not technically an insult but worth sharing anyway)
2. You bootless lungfish, you absolute bloody curdled mothball, you loathesome turdblossoming disaster of genetics! (One of my favorites)
3. Your stupidity level! It's over 9000!
And last but definitely not least:
4. I wish upon you the Seven Terrible Curses! May every toilet in which you crap overflow bountifully! May your floors become a breeding ground of stickerweeds, suspicious puddles, and legos! May every shirt you ever call your favorite be ruined irredeemably! May you always be afflicted with horrible colds on the days you've been looking forward to, including weekends! May your coworkers ever view you as an utter imbecile! May you be extremely itchy, hot, fidgety, and tongue-tied, whenever you try to make a good impression! And may your favorite TV shows always be permanently cancelled after agonisingly huge cliffhangers!

fruit fried fuck

You ding ding