My vote would go to Sigma Chi, due to the "Sweetheart of Sigma Chi" catchphrase. Other orgs might be the fictitious Tri-Lambdas from the "Revenge of the Nerds" movies or of course the "Delta House" from the movie Animal House (I don't think the average person would remember their full name of Delta Tau Chi, though).
Delta Delta Delta is probably pretty well known also, if only for that unfortunate SNL skit.
In short, if a GLO made it onto TV, people will have heard of it.
eta: Rain Man, good call with the Phi Beta Kappa too. I didn't think of that!!
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