The only 24 hour system in the world. That's nothing to sneeze at. Almost no other metro system is even close to the hours NYC pulls. It's sort of unbelievable to me how early metros like Tokyo as just one example close.
It's what "everyone" knows about, moves 5.7 million per week, almost a million per day on average; many who are just visiting NYC have used it.
It hasn't broken down too many times, and is reliable enough that many New Yorkers don't own a car (which IMHO is the gold standard for trust in a public transit system).