Well, in my time, I would have had to say the strongest fraternity was Sigma Chi.
Reasons:
They were not the smallest but not the largest either - i.e. there weren't many "who is that random Sigma Chi?" sightings, as in they were a Sigma Chi and didn't do anything else. (The fraternity that was the largest had this issue.)
They were cute.
They were fun to be around.
They threw good parties with good music.
They had a diverse mix of brothers and many of them were very strong individuals (i.e. famous on campus).
Several of them had beaucoup $$.
They were kind of closer with one sorority but definitely tried to get past it.
Most important - they showed that they truly cared about each other and about their fraternity. I still remember how the brothers talked about their history, their different programs, their alumni - it was with a level of pride that no other fraternity on campus had.
Then again, this is one person's opinion, and female opinions are different than male opinions.