Tittle and poll says it all.
I’ve always been partial to the UK myself. 
The categories are too broad. US slang as an example has many different forms. Are we talking about the Lowcountry or Central Appalachia or Brooklyn or San Fernando Valleyspeak? I like Central and Southern Appalachia with its heavy emphasis on simile and metaphor. “The Devil’s Beating His Wife” and its ilk are enjoyable turns of phrase. Similarly in the UK, Cockney rhyming slang and its many derivatives is pretty cool.
I’m not sure I can even think of an example of Canadian slang
Watch Letterkenny and you’ll get an encyclopedias worth.
Calling that round crap McDonald’s uses on its breakfast sandwiches “bacon”? 
I voted Australia because not only do we have a vast array of our own we blatantly use everyone else’s if they fit the situation.
Listen up daddy-o, me and my moll (the one with the gams that reach to the moon) will shell out lotsa clams if one of you palookas can find better slang than 1920’s / 30’s American Jazz-age slang. It’s the bee’s knee’s I tell ya!
Better than today? I’ll cap yo ass with my 9 while my ho laughs at yo punk ass.
Blimey! Gotta go wiv da UK cockney rhymin’ slang. Nuff said, yeah?
The old toil-and-strife likes that but I never found the appeal.
Aw, dry up, chump! Ya think you’re the big cheese, but yer giving me the heebie-jeebies flapping yer yap ‘bout drilling my keister full of lead with your bean-shooter. Take a powder, pal!
… and, if your flim-flam flop-house floozy giggles at my copacetic caboose, I’ll bust that bim’s beezer till she blows town—pretty peepers, pins and all. Ish kabibble!
Now, excuse me, I gotta go see a man about a horse. Then, I gotta go see another man about a dog … so, I may be tied up for a while. Blimey!
[note: no Dopers were harmed during production of this post]
a clockwork orange
reckon that a bit of fair dinkum straya slang is the ducks nuts.
I’m currently reading a dictionary of UK slang from 1811. They have everyone beat, especially with terms for sex and executions.
Like – dance upon nothing (to hang), Deadly nevergreen (gallows), Earth Bath (a grave), “Fine as a cow turd stuffed with primroses,” hempen widow (one whose husband has been hanged), live stock (fleas), pratts (ass), brother round mouth speaks (a fart).
Not sure I could answer this since there is so much inventive slang in so many times and places.
Kn’oath.
And bodily functions…I’m not sure how far back the idiom goes, but I remember Roger Daltrey’s comment about the superbowl halftime show, “It’s kind of weird! I suppose it gives people time to take a slash and that’s about it.”
I’m trying to envision Tex Avery’s classic “Symphony in Slang” cartoon written for any other country and failing miserably…
Well, everyone keeps appropriating AAVE…
That’s no eel, berk!
('Opefully the orderly daughters won’t hair me for me palare.)