Who is to say that Hulkenberg or Ocon isn't a top driver?

Their own results against other drivers. Hulkenberg or Ocon haven't smoked Perez like Button has.

Villeneuve was able to win the wdc but was utterly destroyed by many drivers.

Hence why he was never considered a great driver, sure he was very good, but he was never put in the elite of Schumacher. Because he wasn't that good.

Schumacher was able to win 2 WDC's against a couple of mediocre drivers

Have you considered the machinery he had compared to those mediocre drivers? He was firmly above any driver of his time. After Senna's death it took the rise of Alonso for a driver of his level to show up again, this is evident by the results against their teammates.

Why do we rate him higher than Vettel?

Because Vettel made a lot of mistakes in the following seasons: 2009, 2010, 2012, 2014, 2016, 2017, 2018. Schumacher made lots of mistakes in: 1999, 2003, 2006. Hardly close.

Yes, Rosberg is arguably on the edge of the best drivers, tbf I dont think he'd look so bad next to Vettel.