Off the top of my head, I think it may be “Imma,” as in “Imma let you finish,” as Kanye West said to Taylor Swift after barging on stage and interrupting her at the 2009 VMAs. “Imma” is obviously short for “I am going to.”
Any other nominees?
TANSTAFL.
(There’s No Such Thing As A Free Lunch.)
If acronyms count.
I’ve usually seen it spelled out as “There Ain’t No Such Thing As a Free Lunch.”
Yes, that works even better.
Commonly heard in the Upper Midwest: “Djeet?” Short for, “did you eat?”
CAPTCHA
Australian - ‘onya’, short for ‘good on you’ - an affirmation of praise, well done, bravo, eg onya Narelle.
“I’d’ve” for “I would have.”
(Letter for letter, it exactly matches the OP’s example.)
Also in New Jersey, and Philly, and Rhode Island, and the Deep South 
Do we count former slang that went mainstream? If so, hard to beat “bye.”
It’s from “God be with you.”
FUBAR -F*d up beyond all repair/recognition/recovery.
Courtesy of the U.S. military.
IMO, slang words and acronyms are two different things, but one I remember hearing in college from a guy ‘not from around here’ (we were in California, he might have been from New England, maybe?) was “squeet” (or maybe it was “skweet”). He was saying, “Let’s go eat.”
“Wanna” (Want to X…) and “Gonna” (Going to X…), in both questions and declarative sentences.
One of my college friends and I (both of us from northern Wisconsin) used this all the time, too. I wasn’t sure if it was a regionalism or not.
POTUS - President of the United States, which was used by telegraph operators.
“Wanna score some 'za?” = “Do you want to go and get some pizza?”
It’s not exactly slang, but “LA” is universally recognized as standing for “El Pueblo de Nuestra Señora la Reina de los Ángeles del Río de Porciúncula”. Far more recognized than the original.
Not the OP, but I’d opine that anything properly rendered in all caps is too easy.
Djeet? =Did you eat yet?
Narp = No.
'squeet = Let’s go eat
Thinkin? =Where are you planning on going?
Pat’s fer za =Pat’s Pizza for a pie.
'sgo = Let’s go.
Was a common conversation when I was in college in Maine.
You left out: “Mungry.” According to my acquaintance (and I don’t think I can say he was a friend since I don’t recall his name!), that preceeded “Squeet” in the conversation.