My vote goes to the Challenger 2 tank as well, and I am an American.
The Abrams is an awesome piece of armour, but it has too many drawbacks for me to consider it "teh bestest". For one, although compact, quiet, and amazingly powerful (3800 ft/lb torque, 1500hp), the turbine motor in the Abrams guzzles way too much fuel for practical use. Yes, the military pretty much has unlimited budgets when it comes to all-out warfare, but wow,
that's a lot of fuel! Mechically, while it's easy to maintain and quick to switch out, British and German diesel engine designs are just as practical in the field.
Secondly, the armor itself is better on the Challenger 2 than any other tank in the world. Americans and Germans get their armor designs based off the British, so that means, despite excellent protection, the Challenger 2 will always be one step ahead. I've read reports of Challenger 2's getting hit with multiple RPG's and 125mm tank rounds and still living to see the sunrise again. It's a tremendously heavy tank, but its crew protection is second to no one in the world.
The gun is the only drawback to the Chally, I think. It's not that the gun itself is weak (it's not), but in concordance with other NATO powers, the British are on their own when it comes to ammunition supply. The Abrams and Leopards both use the Rheinmetall smoothbore, while the French use a GIAT smoothbore (both of which can use the same ammunition, but the French still supply their own). If I'm not mistaken, the Rheinmetall 120mm is the best tank gun in the world, but I'm sure the Chally's rifled gun has no problems punching holes in Russian armour.
The other part of me is just jaded. Approximately 50% of the US Army's yearly funds go torwards maintaining our fleet of ~8000 Abrams battletanks, and I'm sure a HUGE chunk of that is just gas costs. The turbine motors themselves are old, and with age comes higher maintainence costs. We can't just put diesel motors in there either because the engine bay isn't big enough.
Other than that, the Chally2 gets my vote.