A couple of my favourites would be:
"He's got a head like a sun-fucked onion"

My apologies if swearing bothers you

Conversely, please send me your favourite

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Contributions so far:

Shannon Gerard
has offered:
"bitch-wad is great, but it's no crapnuts."

Tavis Stevenson has thrown in:
"i forget where it came from? but a favorite around the stevenson-sheehan camp fire was "face like a sack of smashed-assholes" and someone mentioned "Dill-weed" the other day..... yeah.. take that!

And from Patrick Nadeau :
My grandmother would always tell me to close the door,
adding "it's not an asshole, it won't close by

("Ferme la porte. C'est pas un trou-d'cul, ca s'ferme
pas tout seul.")

Also I like 'ratfuckery', as in "exasperated at the
sheer ratfuckery of it all".

Josh professes a fondness for:
Crap Nuts (to tell you the truth, I think he can do better....)

Dave Thorne (all the way out in Beeutiful BC) adds to the growing pile with:
I like the phrase cock jockey...

Some lass named Brit McKee (a sort of art star at good ol' YorkU) is, I am told, smitten with:
shit gizzard
(just so we are square, the above link is most probably not for the cursing Brit, but some other, way less interseting, but equally unknown (to me) Brit McKee.
Aw, fuckit, she's just a crank who's out to steal my girlfriend

Daryl Vocat: "Yes, THE, Daryl Vocat!
"your mother sucks cocks in hell.... " has always been a favourite of his. Funnily enough, I used to enjoy spouting that curse a fair bit. Also, it is from one of my (and perhaps Daryl's) favourite movies

From the distinguished Gerald Hannon:
My mother always used to say, "wouldn't that frost your nuts!" -- meaning, I think, "wouldn't that discombobulate you!" I had no idea for years that it might have a somewhat unsavory meaning. I was a very naïve child.

Steve Morse "Yes, THE, Steve Morse!
"Oh and the ever popular cumbubble.....use to be a favourite of mine"

From Mina Cicconi:
Douche bag --- such a good one.