There’s no one way to judge prestige for fraternities. Is it which fraternity has the highest average member GPA? Which has the most astronauts, or U.S. presidents, or professional athletes? Which has the highest graduation rate for its members, or the largest endowment? The best leadership training? There no easy metric for measuring prestige.
And even if there was, every national fraternity has amazing chapters at some schools, awful chapters at some other schools, and everything in between. It really depends on the school you’re looking at.
And as the other commenter mentioned, Phi Beta Kappa has not been a social fraternity for a long time.