Who's got the best tank? Most people would say it's the American M-1 Abrams. Their reasoning would be simple; the M-1 has actually fought in two wars since 1991 and handily defeated whatever was sent against it. Tank buffs, however, tend to look more closely at details casual observers ignore. The buffs tend to consider the German Leopard 2A6 as superior to the latest model M-1A2. The Leopard 2A6 has a longer 120mm gun barrel, giving it's shells greater penetration. The Leopard also has reactive armor for the top of the tank, where the latest top-attack missiles seek to penetrate the thinner armor there. The Leopard also has a number of other novel touches, like a video cam facing to the rear of the tank, and hooked up to a screen in the drivers compartment. This allows to driver to go into reverse more quickly and confidently. Backing up quickly is a frequently used combat maneuver. The Leopard also has a diesel engine, rather than the fuel guzzling gas turbine (jet engine) of the M-1. Thus the M-1 has a little more zip, but the Leopard gets much better gas mileage.
But a tank does not stand by itself. It is part of a combat force, and the most important component is the crew. In this department, the M-1 has several advantages. Most importantly, American tank crews have had a lot of combat experience since World War II, German crews have had none. While German training is good, they are still using conscript crews, while U.S. tankers are all volunteers and in service longer. American combat doctrine has also developed more rapidly than Germany's and currently makes heavy use of the battlefield Internet and superior situational awareness. All of this makes an enormous difference. A tank is not the sum of all it's parts, it's only as good as the system it operates within. Here the M-1 has a big edge. Moreover, the Americans get an additional slight edge because of their willingness to use depleted uranium in their composite armor, and tank shells. Then again, if the U.S. and German switched tanks, the Leopards with American crews would be superior.
The other tanks in the "top ten" are remarkably similar. Most have composite armor, and often reactive armor as well. All have guns similar to the M-1 and Leopard's 120mm smoothbore. The British Challenger 2 is usually ranked third. But, again, because the British armor units have had combat experience since World War II and use volunteers, they have an edge. Because the Americans have more proven combat technology, the M-1 would still be first, but the Challenger 2 would be second and the German Leopard third.
Things really get interesting when you try to fill the fourth place slot. There are a lot of high tech tanks out there. The French have the LeClerc, the Japanese have the T-90, the South Koreans have the Type 88/120 and Israel has the Merkava 4. Again, the edge should go to the tank that has the best crews and the most combat experience. That would be the Merkava 4. While lacking a lot of the gadgets of the other tanks mentioned above, the Merkava has an edge because of combat experience and crews with years of working together. Although most Israeli tank crews are reservists, many of the troops have combat experience and the crews often stick together for decades. This makes for very effective crews and tank units.
Fifth place belongs to the South Korean Type 88/120. This tank was developed by the same people who created the M-1. Some call it the "Baby M-1", as it is a bit lighter than the M-1 (51 tons versus nearly 70 tons), but otherwise uses the same design principles. Most important is the fact that the South Korean crews know that they have a deadly foe just to the north. This provides a little pucker factor to the training, which is run using a lot of American techniques.
Sixth place is tricky and is a toss up between the French LeClerc and the Japanese Type 90. The edge goes to the Japanese tank. Both vehicles weigh about the same and use similar weapons. But the Japanese have better electronics and crews that have been together longer. Plus, all things considered, I be a little more fearful of a bunch of Japanese crews in their Type 90s than French crews in their LeClercs.
Seventh place, by default, goes to the LeClerc.
Eighth place would be the Russian T-80UM2. This tank uses a lot of new protective technology (to detect and defeat anti-tank missiles), several armor systems and lots of electronics. Unfortunately, the workmanship is slipshod and the crews mostly conscripts and poorly led.
Ninth place goes to the new Chinese Type 98. This is another of those "improved T-72s." Lots of improvements, though, many of them similar to what's found in the Russian T-80UM2. The workmanship on these vehicles is a little better than on the T-80UM2, but the Chinese don't have as much experience building tanks. This has shown itself in the numerous technical glitches that have shown up. The Chinese are moving to volunteer crews and more intensive training.
Tenth place goes to the Russian T-90, which is actually an upgraded T-72. Not as effective an upgrade as the T-80UM2 or the Chinese Type 98.
Most of the remaining tanks in the world are Russian T-72s and T-55s, and US M-60s and M-48s. China builds clones of these Russian tanks, and other countries build variations on the T-72 and older British tanks. The M-60s, with the latest upgrades (thermal sights and computerized fire control systems) and well trained crews could be contenders for the 8-10 positions. But all those T-72s and T-55s serve largely as targets. However, as experience in the Arab-Israeli wars and World War II amply demonstrated, technically "inferior" tanks with superior crews will rule the battlefield.