A bit more about the evolution of the tank

.

I mentioned a bit of the tank evolution history recently, maybe a more comprehensive approach is worth a try as well.

First part; the important milestones in tank history:

Mark I

Leaving armoured cars and a few prototypes aside, the first real tank was the British Mk.I tank.

Two other notable WWI tanks were the French FT-17 (first light tank) and the British Medium Mark A "Whippet" (first tank with emphasis on speed).

FT-17

Medium Mark A Whippet

Independent

The next milestone was the British A1E1 Independent of 1926, a prototype for the multi-turret heavy tank strain that was popular till it failed in 1939. The multi-turret approach was a dead-end, although it could be said that the BMP-T (see end of blog post) revived the basic idea.
This tank prototype also had the speed (30 km/h) which became typical till the early 30's.

Char B-1bis (photo Igor Kurtukov)

The Char B-1bis (1937), a late version of the Char B-1, was the first shell-proof tank. Tanks had previously only been protected against bullets, fragments, small explosions and small calibre cannons.
The Char B-1bis featured sloped armour and an armour thickness that protected it well against the anti-tank guns of its time and light field artillery.

Pzkw III

The next milestone would be the German Pzkw III (1937), in part as representative for the German tank development of the 1930's that placed a great emphasis on command & control (Führbarkeit). The combination of voice transmit/receive radio, three-man turret and commander's cupola proved to be a revolutionary 'hidden value'. It had a top speed of 40 km/h which was representative for its period and a torsion bar suspension (this became the dominant form of suspension post-WW2).
The Pzkw III-based StuG III defined the tank category of assault guns.

early T-34/76 model

The T-34/76 was a famous milestone in 1940 because it had a great balance of firepower, mobility and protection in 1940-1942.  It can be considered as the first real main battle tank.
Its sloped armour was no innovation, but became famous and established as design feature until the rise of composite armours (which are internally sloped).The T-34 was a low-quality design in its details despite the superficial genius of the design.
The T-34/76 spawned a long lineage of Soviet tanks (T-34/76, T-34/85, T-44, T-54, T-55,  T-62) with great longevity.

Pzkw VI "Tiger"

Next I'd like to mention the Pzkw VI "Tiger" (1943) as the prototype of the new breed of heavy tanks; with a focus on firepower and protection. The KV-1 tank of 1939 would take his place if it had been equipped with a more powerful gun. I think the Tiger is a better prototype for the new heavy tanks because the KV-1 pre-dated the T-34 (same firepower) by few months.

The Tiger's overlapping and interleaving road wheels were a milestone for other German WW2 tanks and reduced the MMP so much that ground pressure was no extraordinary problem despite the tank's heavy weight.

Leopard

The next step was probably the Leopard 1 (1965). It was designed with highly mobile warfare and the lack of protection technology to meet the shaped charge threat in mind. The result was a good tank which did superficially excel only with its mobility. The 'hidden values' of very quick and easy maintenance and good durability are the reason for it being a milestone.

T-64

The Russians finally departed from their successful T-34 and KV-1/JS formulas with the T-64 tank at almost the same time (1966). It featured a revolutionary autoloader and a very large calibre (125 mm) gun combined with good protection (sandwich front armour) and mobility (comparable to Leopard  1). Its low silhouette and weight (38 tons) were achieved at the price of some 'hidden values' (such as good ergonomics). It was generally not easy to maintain or quickly repaired.
The T-64 became the defining design for the later Soviet/Russian tanks designs (T-72, T-80, T-95).

The 70's saw the development of so-called "Burlington armour" and later "Chobham armour", two  successive composite armour technologies from the UK which were good at defeating both shaped charge chemical energy penetration and subcalibre kinetic energy penetrators. The weight and bulk of such armour limited its application to the front, but it allowed nevertheless for the next milestone:

Leopard 2A4

The Leopard 2 (1979) tank found again a great balance of mobility, firepower and protection, albeit at the cost of having the weight of a heavy tank. It lead a wave of new Western (and far Eastern) MBTs, all of which slightly varied from the formula.

The British Vickers Mk.7/2 (1985) combined the Leopard 2's hull with the turret of the Vickers Valiant (1979, aluminium basic armour + Chobham). These two Vickers (proto)types of the 80's made use of modular armour (which became famous with the much later Merkava Mk IV) and the Mk. 7/2 also had some extras that led the way to the electronics-heavy tanks of the 90's.

The Soviet Union and Israel pioneered the use of  reactive armour in the earl 80's: ERA is made of plates with some explosive behind that interfere with long rod penetrators and shaped charge jets because of their secondary explosion outside of the main armour. This yielded no real new tank design, but many upgrades. ERA was entirely external and requires no wiring - perfect for upgrades. Germany, Britain, UK, France, Japan and South Korea have been reluctant to adopt this technology, supposedly because of the hazards.

Leclerc, photo by Daniel Steger

The French were late with their Chobham generation MBT by a decade and were therefore able to integrate many advanced electronics into their first Leclerc (1990) MBT models. I think it's notable not only for the great amount of electronics but also for its commander's independent thermal viewer which pre-dates the M1A2's CITV. Most Western (including South Korea, Japan) MBT types have seen upgrades with extensive electronics and sensor packages by now. The intentionally low-tech Ariete  MBT from Italy is an exception.

Future tank milestones?

The future path of tank design is unknown. We know that we've made substantial technology advances since the last milestone that should justify new designs if the Cold War hadn't ended long ago. Many of these have been incorporated into legacy tanks as upgrades, for many advances were 'hidden value' advances such as better electronics or more durable track links.

A new milestone for tank development could be found in a return to more moderate weights (such as the Japanese did with their Type-10 (2010, 44 tons) MBT or a drastic change in armament, probably towards a higher rate of fire with smaller shells as in the Russian BMP-T tank (2005) which was meant to supplement MBTs but still capable of doing essentially the same as a MBT only with different weapons.

Type 10

BMP-T

Externally mounted guns (as in many T-95 speculations and the modern Puma IFV) have been a supposed MBT development milestone in the waiting for decades. Their greatest theoretical advantage (small frontal turret silhouette)  doesn't seem to be a practical advantage because of all the sensors and other equipment that get attached to turrets.

The concept of an armoured 8x8 vehicle became very extremely fashionable in 1999 because of Gen. Shinseki and the Task Force Hawk debacle. The U.S. Army feared to be "irrelevant" in the age of cruise missile diplomacy and diagnosed its lack of quick deployability by air as the culprit. The "Stryker" and FCS programs were meant to provide the (interim and full) remedies. The easy shooting of poorly-employed Iraqi low-tech tanks in 1991 had convinced many people that even a 105mm gun on a truck would be able to do the same. This became the Stryker MGS.
8x8 vehicles were never capable of really replacing MBTs simply because their mobility became unacceptably poor at heavy weights, and high protection levels add much weight.
This 8x8 craze (many small countries followed this fashion) and the associated hype about electronics and information supposedly replacing armour were dead-ends.


The U.S. Army disintegrated/aborted its FCS program and has also re-started its newer GCV program. They have doctored on one new tank program after another since the early 90's, but all this activity with billions of dollars spent did not yield a single new operational MBT or IFV design.
 
The Russians doctored a lot on their T-95 MBT since the 90's. It seems to have been abandoned in favour of upgrades for proven types. There will apparently also be an altogether new MBT (or a heavy tank family?) design.

Most actually new MBT designs come from East Asia nowadays. There are also very large production runs and inventories. It's reasonable to expect the next milestone from the non-traditional tank producing countries of PR China, South Korea and Japan. Most of their new designs still seem to stick to the basic recipes of the T-64 and Leopard 2 milestones, though. The most notable exception is the aforementioned Type 10.
Maybe I missed some "hidden value" advances, though. It's not exactly easy to learn about details of their new tanks.

S O

P.S.
Sum: 4 German, 3 UK and French, 2 Russian milestones.
I really tried hard to find a U.S. milestone, but there's none. The "Skeleton tank" came too late. The Christie suspension was inferior to torsion bar suspension (once metallurgy became good enough) and it was never fully employed in a really successful or good tank. Gun stabilisation (first seen in 1940') didn't turn the M3 Stuart into a milestone by itself. The Sheridan and M60A2 were dead-ends without followers. The M1A2 appeared after the Leclerc.
US. tanks were typically mediocre (even though some hyping began when Clancy wrote about the Abrams, a design that actually looked inferior in the trials and was ordered as a kind of subsidy to its struggling maker Chrysler). Germany cooperated once with the U.S. on a tank design (MBT 70); it turned into a combination of dead-ends and pioneered merely unacceptable per-unit prices.

The Merkava was excluded because it's an exotic design which found no copycats (save for a handful of post-Cold War Eastern European prototypes).

The late Centurion was a good design, but no milestone for anything but the 105mm L7 gun.

The Panther (medium/heavy WW2 tank with high speed) was no real milestone in tank design either.

.