why is pm the top house?

The top house for you is what fits best for you and where you feel at home. PM is highly regarded because they are well rounded, have very strong pledge classes consistently, they generally stay out of controversy, they win a lot of stuff and awards like homecoming and greek excellence, they keep programs going that create value for members throughout their college career (not just freshman and sophomore parties), they have leaders all over the place, big in SGA, and overall reputation. Their gpa might have slipped but it is still good. Other houses are great too. It is all subjective

I think @jj summed it up pretty well
Also, a lot of girls are turned off by them during rush but a lot of girls LOVE them during rush so they are able to be selective during rush (which I’ve always thought is a lot of what makes a top house).
On campus history is a big part of what makes a house a “top house”, which is why OR is always ranked above NR on this site even when objectively a lot of NR houses are amazing.

What makes them and axo stand out among the old row houses is they probably have the most out of state girls and the don’t have a big drop off from the top to lower end of their pledge classes. AXO is not as well rounded though but they kill it on the grades. This hurts them on looks and social relevance. PM has a lot of smart girls but they will definitely take lower grade pnms that bring a lot of other stuff to the table. AXO seems more rigid on that. That is why to me pm might be seen as a little better overall. Like a post above says though, there are many great houses and to me the top half dozen are basically at the same level and it comes down to fit and subjective