I was looking to grab a slice before catching a train at Penn Station, and stumbled on sliceharvester.com, the blog of a guy who claims to have tried every pizza-by-the-slice place in Manhattan. He said Pizza Suprema, right across the street from the station, was one of the best. I can't claim to be the connoisseur he is, but it sure was good. Perfect crust, and sauce with just a touch of sweetness. Try the upside-down slice, which puts the sauce center stage, but also get a plain slice to experience how good something so simple can be. I was a very satisfied train rider.