Ahem.
"Y'all" is a perfectly legitimate contraction for "you all," which is a clear and genteel manner of indicating the plural pronoun "you." As a contraction, the apostrophe is necessary in the United States to replace the omitted letters. (In the UK, apostrophes are sometimes not used in contractions. I'm not sure I like that.)
"You guys" is crass, unless one is speaking exclusively to a group of guys. I am not a guy, and I don't respond to people who address me as one.
"You'ns" sounds moronic, but it's really oxymoronic. It combines the plural "you" with "ones." Kind of a plural multi-singular.
Any questions?
