I’m kind of partial to the English: ‘Sad sod’
‘Balls-sack’ is still popular here, in the Netherlands. [klootzak]. A strange variant is: ‘Balls-violin’ or maybe ‘Balls-fiddle’ [klootviool].
A very old and quaint one is: ‘You backwards Gladiola’. [Achterlijke gladiool]
When something’s really bad some say: ‘That’s c*nt with pears’ [kut met peren]
Don’t ask me why.
I’m partial to “fuckwit” and “shiteyes” as well as the British insults “toerag,” “prat,” and “wanker.” For an exasperated-sounding expression to use on a friend, “dipshit” still works, but is too funny to be hurtful.
My friend’s little sister loves calling everyone big-butt. It works everywhere!
“Pillock” is pithy and effective.
I don’t know if it’s hip, but I like to call people Piñata because I want to beat them with a large stick.
sidle, you like school in tha summertime!
I’m a big fan of “donkey-feltcher”, though “douchebag” is a trusty standby.
If you’re writing it on-line, Yuor teh big kok or owned is always fun.
LOL! 
As far as insults go, I’m highly partial to “grotty little wanker”, but I haven’t really had the chance to use it, and I’m not sure I can pull it off anyway what with my American accent.
Someone in a semi-recent pit thread used “fuckwicket,” to the approval of many posters. It does have a certain ring to it.
Allow me please to introduce - for the first time ever - the insult my friend coined at school, that he filled in as a crossword puzzle answer and consequently got banned from the school library because of it:
Cuntfood.
Bagnuzzler
Cockgobbler
For some reason I hadn’t thought of this when I posted here earlier - some of my favorite insults are ‘cunt’, ‘cuntface’, ‘cuntbag’, etc. They work well for me.
Heh!
Loving some of these.
Honorable mentions to fuckwicket and assratchet and assmonkey.
Fucktard is also one of my personal favorites, but I’m afraid to use it online, with the suffix and all.
Safer to use politically correct offensive terms, yes?
Fucknozzle.
“Fick dich ins Knie” and “leck mich am Arsch” are good for a Teutonic flavor.
Obviously, sidle rides the short bus to school.
You mean something like this?
The folks who produce these t-shirts clearly need to use that illustration instead of the more standard version!
Some of my favorites:
Knuckle fucker
Ass Jockey
Ass Pirate
Ass Goblin
Dilhole (when joking around with my nieces and nephew)
Nob Gobbler
Ball Sucker
Wanker (in my best pseudo-british accent, of course)
For the ladies:
Whoore (heh, you’re not the only one Anitgen)
Dirty Sloot
Slunt
Slag
Cock Junkie
Sloppy c*unt (I’ve got to be supremely pissed off for this one)
When encountering someone with foul breath:
“God DAMN! Have you been giving rim jobs down at the homeless shelter???”
or:
“Jesus Christ, did you just eat a hot shit sandwich?”
From a man to another man:
“You’ve got the softest chin I’ve ever laid my balls on.”
I usually enjoy calling people:
Ass wipe
Knob head
(under my breath of course) 