What is the title of the paper with the most authors?
paper refers to paper.Id; paper with the most authors refers to MAX(PaperAuthor.PaperId)
SELECT T2.Title FROM PaperAuthor AS T1 INNER JOIN Paper AS T2 ON T1.PaperId = T2.Id GROUP BY T1.PaperId ORDER BY COUNT(T1.PaperId) DESC LIMIT 1