Sandman is a clear winner, mmm... Neil Gaiman. While Cerebus is certainly a worthy contender. Some of the Batman stuff is truly excellent.
I never really liked Spawn (although I only really read the first twenty or so issues so it may have changed), the fact that MacFarlane couldn't write for tofee and thus had a continuous stream of guest writers meant the story was permenantly in a state of flux and never really fulfilled it's potential.