I'm going to ignore for a moment analysis of tactical achievements on the battlefield. I think the central question is whether or not can we see the chain of subsequent history unfolding without the man in question.
1) Was there another agent that could have united Greece, thus ending the Classical Age of the polis and beginning the Hellenistic era of larger land masses ruled by god-kings?
2) If Alexander had not brought token measures of Hellenism to the Orient, could Rome have conquered and maintained what became its Eastern half?
3) Would Roman culture have developed as it did without the incorporation of a Hellenistic East standing in Alexander's shadow?
4) Would Christianity have developed as it did, if at all, without aforementioned Hellenistic East subsumed into Rome?
5) In the sum of things, how would Western Culture be different?
Everything is speculation, but it's hard for me to envision history as we know it without Alexander. And from that perspective, as a mover and shaker of world events, Alexander might be called "great" whether you agree with his actions or not.
Those of us who appreciate the "great man" theory of history may simply appreciate in Alexander a man who sought glory and won. The amoral pagan in me finds comparisons to recent totalitarian leaders beside the point. To go down in history, whether in fame or infamy, might be argued by some to be better than dying unknown and forgotten to the ages.