Leopard 1
Entered service in 1965 and remains in use in nine countries today. Focused on firepower with L7 105 mm gun and cross-country performance.

Leopard 2
The main battle tank developed by Krauss-Maffei in the early 1970s for the West German Army. First entered service in 1979 succeeding the Leopard 1. All models feature digital fire control systems with laser rangefinders, a fully stabilised main gun and coaxial machine gun, advanced night vision and sighting equipment. The sophisticated computer system provides the ability to engage moving targets while moving over rough terrain.

Leopard 2 to Leopard 2A4 have vertically-faced turret armour

Leopard 2A5 known as the ‘improved’ batch have arrowhead shaped turret, and armour tailored to a given threat scenario.

Leopard 2: the baseline Leopard – the first series manufactured version.

Leopard 2A1: has improved armour protection. The two most notable changes were the modification of the ammunition racks to be identical to those in the M1 Abrams, and redesigned fuel filters that reduced refuelling time.

Leopard 2A2: This modernised first batch can be recognised by the circular plate covering the hole where the cross-wind sensor for the fire-control system was removed. The original sights were replaced with thermal sights. The upgrade also included the fitting of filler openings and caps to the forward hull fuel tanks to allow separate refuelling, as well as the addition of a deflector plate for the periscope and a large cover plate to protect the existing NBC protection system. The tank was given new five-metre towing cables with a different position.

Leopard 2A3: fitted with improved communications equipment (digital radio sets), improved parking brake and the ammunition reloading hatches were welded shut.

Leopard 2A4: fitted with new digital fire control system and improved turret with flat titanium/tungsten armour. It was the most widespread version of this tank. All the previous models were upgraded to this standard.

Leopard 2A5: a radically improved version introduced in 1990. The A5 introduced an arrow-shaped, spaced add-on armour to the turret front and the frontal area of the sides, designed to defeat shaped charge weapons and take out the kinetic energy of APFSDS rounds. There were also some improvements in the main armour composition. The tank interior received spall liners to reduce fragments if the armour is penetrated. The frontal “heavy” third of the side skirts was replaced with a new, stronger type. The commander’s sight was moved to a new position behind the hatch and it received an independent thermal channel. The gunner’s sight was moved to the turret roof as opposed to the cavity in the front armour in previous models. A new heavier, electronically operated sliding driver’s hatch was fitted. Turret controls went all-electric, increasing reliability and crew safety, and producing some weight savings. The gun braking system was improved to prepare for the later mounting of the new L55 gun tube and to enable firing of more powerful ammunition, such as the DM-53 APFSDS.

At that time the Leopard 2A5 was referred as the best main battle tank in the world.

Leopard 2A6: another radically improved version with improved armour protection and a longer Rheinmetall 120 mm L55 smoothbore gun. The L55 is 25% longer than its predecessor, providing higher muzzle velocity, which gives better range and penetration for APFSDS kinetic energy rounds.

The Leopard 2A6M is a version of the 2A6 with enhanced mine protection under the chassis, and a number of internal enhancements to improve crew survivability. Trials in February 2004 demonstrated that, with the new armour package, Leopard 2 tank crews could survive the detonation of an anti-tank mine under the tank without suffering any injuries. Replacement of the hydraulic drive with electrical, and the drivers seat, which is traditionally bolted to the hull, is now suspended by straps so it moves freely.

Leopard 2A7: a proposed upgrade package for existing MBTs, which includes additional armour, additional cameras for long-range surveillance, advanced command and control equipment and some other improvements.

(source: http://www.military-today.com/tanks/leopard_2.htm)

(source: https://tanknutdave.com/the-german-leopard-2-series )