I would go Leopard, Abrams, then Challenger. Even according to Janes who is a quite reliable source, the Leopard is superior to the Abrams, but the Abrams has a vastly superior supporting crew. The US Air Force could achieve Air Superiority in only a matter of weeks, if not days, over any other Air Force in the world, IMO. Tank to tank engagements; while still a necessity of war, are not as common as they once were. Close air support would mean that the Abrams may be used as use a spotter or even bait, meant to bring the enemy out into the open where they will be caught in a turkey shoot.