Like The Hulk, Apocalypse can increase his own strength. But Apocalypse can do so with possibly no upper limit and he can do it at will rather than having to rely on his own rage. Besides that, Apocalypse can reform himself in any shape (down to a molecular level) and even absorb the energy of others. So whatever strength The Hulk can throw at him, Apocalypse can simply take for himself and use right back. We don't even have to get into the fact that he's another character that The Hulk has lost to in the past. This one was never even a contest.