KDAmbular: i have the opposite problem!!!! there are only four sororities and four chartered fraternities so how could i pick two and leave out one or two?
in the past i would have definitely said that theta's and adpi's are most often seen together, but more recently we've been starting to spend some more time with the kappas and axos.... we have quite a few panhellenic houses/roommates this coming year... i honestly could not choose for myself because i know some amazing and special women from each chapter.
as for fraternities, i can say without a doubt that we spend the most time with the delts and of the sisters dating fraternity guys the majority are delts... though depending on the day of the week i'm sure certain sisters' "favorites" could change to sae, ox, or phipsi
personally i know more delts and saes than oxs (since they reorganized) or phipsis (love ya davi-ka
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randomly, i think that nationally, maybe my faves of those on campus might be kappa and phipsi, for their values, etc.