Well, if you include TUSK as armor kit for the Abrams, then you also have to include the different Theatre Entry Standards (TES) armor kits (three versions at least) of the Challenger 2. The base armor however was most likely not upgraded.
The Leclerc is not geometrically more efficient. It could have been, if it's armor layout wasn't designed so badly. The Leclerc trades a smaller frontal profile for a larger number of weakspots. It uses a bulge-type turret (no idea about the proper English term), because otherwise a low-profile turret would mean reduced gun depression (breech block hits the roof when firing). There is bulge/box on the Leclerc turret roof, which is about one feet tall and located in the centerline of the turret. It is connected to the interior of the tank, as it serves as space for the breech block to travel when the gun is depressed. With this bulge the diffence between the Leopard 2's and Leclerc's roof height is about 20 milimetres.
The problem with this bulge is, that it is essentially un-armored (maybe 40-50 mm steel armor); otherwise the Leclerc wouldn't save any weight. While the bulge is hidden from direct head-on attacks, it is exposed when the tank is attacked from an angle. Given that modern APFSDS usually do not riccochet at impact angles larger than 10-15° and most RPGs are able to fuze at such an angle, the Leclerc has a very weakly armored section that can be hit from half to two-thirds of the frontal arc and will always be penetrated.
The next issue is the result of the gunner's sight layout. While it is somewhat reminiscent of the Leopard 2's original gunner's sight placement for some people, it is actually designed differently. The Leopard 2's original sight layout has armor in front and behind the gunner's sight, the sight also doesn't extend to the bottom of the turret. On the Leclerc things are very different, the sight is placed in front of the armor and this reduces overall thickness. This problem has been reduced by installing another armor block in front of the guner's sight, but it doesn't cover the entire crew.
The biggest issue of the Leclerc is however the gun shield. It's tiny, only 30 mm thick! Compared to that the Leopard 2 had a 420 mm gun shield already in 1979. The French engineers went with having pretty much the largest gun mantlet of all contemporary tanks, but decided to add the thinnest gun shield for protection. They decided to instead go for a thicker armor (steel) block at the gun trunnions.
Still the protection of the gun mantlet seems to be sub-par compared to the Leopard 2 (420 mm armor block + 200-250 mm steel for the gun trunion mount on the original tank) and even upgraded Leopard 2 tanks. The Abrams has a comparable weak protected gun mantlet, but it has a much smaller surface. The Challenger 2 seems to have thicker armor at the gun, comparable to the Leopard 2.
Also, the Leclerc has longer (not thicker) turret side armor compared to the Leopard 2 and Challenger 2, because the armor needs to protect the autoloader. On the other tanks, the thick armor at the end of the crew compartment and only thinner, spaced armor/storage boxes protect the rest of the turret. So I'd say:
Challenger 2: a few weakspots, but no armor upgrades to the main armor Leclerc: a lot of weakspots, but lower weight and a smaller profile when approached directly from the turret front M1 Abrams: upgraded armor with less weakspots, but less efficient design (large turret profile and armor covers whole turret sides) So if you look for a tank that is well protected, has upgraded armor and uses the armor efficiently, the current Leopard 2 should be called best protected tank.