Firstly, it seems utterly fallacious to think that either of these great generals would have allowed themselves to be brought to battle on an empty grassland against an opposing general whom they respected.
It seems that people are assuming that the two guys would only be able to use the troop types that they used historically, in which case I would consider it a win for Alexander. The Carthaginians would have no answer to the incredable destructive power of a well trained and commended Phalanx. The only real weakness the Phalanx had was a lack of Maneuverability (sp?) that was exploited by several Roman generals, but Alexander would have countered this by wise use of his hypaspists. The Carthaginian armies were simply not sufficiently well trained to stand against them IMO. If Hannibal had a free choice of troops like Alexander's phalangites or Rome's legions, then he would probably win in a single battle situation. If they fought a full campaign then Alexander would probably win due to Hannibal's major weakness - allowing his supply lines to be cut due to lack of support from his political masters.
Alexander is no strategist and "conquer everything in sight" is not a strategy.
'Conquer everything in sight' (and incidentally, Alexander conquered plenty of places that were out of sight) is not possible without a very deep understanding of strategy.
A real strategist would've consolidated his Empire instead of trying to conquer India. He could rule for another forty years from Babylon, creating an enormous, long lived Hellenistic Empire.
Alexander did consolidate his Empire massively. Before his conquests the Egyptians, Phoenicians and Greeks were in a state of near rebellion against the Persians, but they behaved themselves once conquered by Alexander. Taxilas and Porus were at war with each other in India before Alexander came along, but they co-existed fairly happily as Satraps after Alexander annexed one and defeated the other.
It wasn't his campaign in India that weakened his Empire, it was his premature death. And although that is
probably linked to the Indian campaign, there is no definate evidence that he would not have died anyway. If you look at the amazing stability of the Seleucids, Bactrian Greeks and Ptolomies then you will realise that Alexander laid the framework for probably the biggest cultural assimilation ever known - it was only his premature death (and later Craterus' premature death) that stopped the Empire from staying together, not any lack of ability or foresight.
My 2 cents there. BTW, look at my scenario for Civ 3 if you're interested in Alexander's campaigns.