Manhattan is home to some of the country's oldest and most celebrated pizzerias, but the great metropolis also holds many mediocre slices. Colin Hagendorf has tasted it all.

Mr. Hagendorf, a 28-year-old Brooklyn resident, may know New York-style pizza more exhaustively than any other living soul. During a 2½-year quest, he has sampled nearly every pie sold by the slice in Manhattan. The feat—involving 362 slice joints—is unmatched by any modern-day enthusiast, according to local pizza experts.

"It's...