You guys are silly. Both Ali and Tyson had ways of hitting you but not getting hit themselves. Another reason why they're two of the greatest.
Having said that, I think Tyson's defense was better. You can throw Ali off his game if you counter his jab or feint him out. He kept his hands low too, so you can catch him against the ropes. As for Tyson, the peek-a-boo style had few flaws but Tyson was so fast that it didn't matter. He was drilled into becoming the perfect fighting machine.