Which is the better tank; the M1 Abrams, or the Leopard 2?
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If cost is no object, Challenger 2.
Short debate. 
The guy who responded makes some good points, but this struck me as amusing (emphasis mine):
But earlier he said that the cannon has a 2-1/2 mile range. 
I haven’t heard much about that one. Just that it exists, and that it’s a good tank.
The Challenger 2 is largely based on the Challenger, which still holds the record for the longest confirmed kill (~5km) by some distance. Armor is basically the same as the Abrams/Leopard.
Only one Challenger 2 has ever been lost in action, and that one was destroyed by another Challenger 2. On the other hand, there are less than 500 of them, so the odds of one being destroyed are much lower.
In terms of effectiveness for unit cost, the Abrams wins hands down - so many have been built that it’s nearly half the price of a Leopard or Challenger.
The differences between the three tanks in terms of technical performance aren’t very significant.
The history of armored warfare suggests that HOW tanks are used is much more important than small differences in technical specs.
Concur. Also, at closer ranges, the penetrator would probably go through either tank’s armor, and without deviating much in its short flight path – making victory largely a question of who fires first.
Actually, even at close range the Abrams and C2 would need to fire multiple rounds to destroy other Abrams and C2s. Not sure about the Leopard.
During the (first) Gulf War, only one Abrams was lost to enemy action. However, several were damaged beyond quick repair during Allied advances, and the other tanks sent to destroy them so they couldn’t be captured by the Iraqis invariably had to fire multiple rounds even at point-blank range to ensure they couldn’t be recovered. These were already heavily damaged tanks with hatches open, mind.
Similarly, he sole Challenger 2 loss was to (accidental) friendly fire, and the tank was only destroyed because the incoming round struck the open turret hatch and the ensuing explosion caused its own ammunition to explode.
For now it appears that Generation 2 Chobham armor development is way ahead of anti-tank munitions development.