Ok as I had nothing better to do, I compared all the tanks as best I could using army-technology.com. And I would say the best tank does not belong to the Americans, Brits, or Germans. Though its hard to compare the American tank since its the only one not measured in the metric system, though I know enough about the comparison rate to insure it is not the fastest or the one with the most range. The M1A2 is so close to the Challenger they aren't worth comparing.
The Leopard seems to be the fastest with most range though it doesn't say much about its armor other than its made so the fragmentation cone is narrowed with its current armor. I think all the others have advanced enough armor to not allow a fragmentation cone at all for the most part.
The Leclerc had the most impressive targeting sytem. The digital fire control system allows the gunner or commander to select six different targets to be engaged in just over 30 seconds. The system's digital computer allows realtime treatment of data from the tank's sensors and sights. It also has a laser warner, missile warner, and infrared jammer. the Leclerc and T80U semi-automatic loading with the Leclerc able to fire 12 rounds per minute and the T80U able to fire 8 rounds per minute. The Leclerc though had a small small payload for ammunition.
The T80U is very similar to the Leclerc. The T80U has the largest gun at 125mm and the gun can also launch anti tank guided missiles at tanks or even lowflying helicopters. The T80U has 28 round carousel clips and a 48 round payload. The T80U has the same countermeasures available to it as the Leclerc and one less 7.62mm gun than the M1A2.
The Merkava 4 is similar to the T80U except with 2 7.62mm guns and a mortar instead of a .50cal gun. It has the typical 120mm main gun it carries the same amount of ammo as the T80U 48 and has 10 round clips that get fed into the main gun. It's fire control system is able to lock onto moving targets even while it is on the move including helicopters. The tank is protected against a range of threats, including air launched precision guided missiles and advanced and top attack anti-tank weapons. The underside of the hull has been fitted with additional armour protection against mines.
SO I would say its between the T80U and the Merkava 4 only because the Leclerc wont do that well under French command. If it were under any other country's command it would also be tied.