top tier?

Seeing a lot about tiers on this website which I didn't realize was such a thing at umass. What makes a frat or sorority top tier? Is it looks, best house, best parties, most parties, philanthropy, looks of the girls they hang out/hook up with, brotherhood, athletics, academics, chapter size? I feel like there is no way the top houses are the best at all of this and was wondering if top tier is really more who girls perceive to be the best or if it means the all around best house to join for a good college experience