While I really like most of Frank Gehry's buildings, I think it is hard to select anyone as the "greatest" architect of our time. For instance, being a Chicagoan, Mies Van der Rohe is close to my heart (though he did works in many cities). But maybe he (like me) is too old.
http://s3.amazonaws.com/architecture-org/files/modules/farnsworth-house-eric-allix-rogers-04.jpg
http://s3.amazonaws.com/architecture-org/files/buildings/chicago-federal-center-03.jpg
http://www.chicago-photographs.com/wp-content/uploads/2014/12/photo-van-der-rohe-iit.jpg