CL-USER> (let ((lists '((1 2 3 4 5)
                        (1 2)
                        (1 2 3 4)
                        (1)
                        (1 2 3 4 5 6 7 8)
                        (1 2 3 4 5))))
           (alexandria:extremum lists #'> :key #'length))
(1 2 3 4 5 6 7 8)