The M1A2 Abrams main battle tank is arguably the best in the world. Yeah, Russia is generating some hype for the Armata family of tanks, but the Abrams is combat-proven and very hard to kill.
View from an Abrams tank of its 120mm main gun firing. As powerful as this gun is, it can't reliably kill an Abrams. (U.S. Army photo)
How hard? Well, in his 1994 non-fiction book, Armored Cav, Tom Clancy recounted a tale of how an M1A1 Abrams got stuck in the mud during the ground war of Desert Storm. It was then set upon by three tanks, Iraqi T-72s specifically. A round fired from roughly a thousand yards away bounced off, and the Abrams responded by blowing the T-72 that fired it to bits. A second round fired from 700 yards, bounced off, and the offending T-72 was blasted. The third T-72, at a range of roughly 400 yards, fired a round, which left a groove in the armor of the Abrams. It, too, was destroyed by a shot fired through a sand berm. These were, supposedly, Russia's state-of-the-art tanks.
Then, when help arrived, and the tank couldn't be freed from the mud, a platoon of Abrams tanks tried to destroy it. After several rounds, they detonated the onboard ammo, but the blow-out panels functioned as designed. Then, when the tank was retrieved from the mud, they discovered that it was still functional. The only issue? A sight was out of alignment.
So, what would it take to reliably destroy an M1 Abrams? Well, someone at quora.com asked what would happen if an Abrams was hit by a round from a 16-inch gun on an American Iowa-class battleship.
A bow view of the battleship USS Iowa firing its Mark 7 16-inch/50-caliber guns off the starboard side during a firepower demonstration. (U.S. Navy photo)
The 16-inch armor-piercing rounds fired from the battleship weigh in at 2,700 pounds. The 120mm rounds fired at that Abrams stuck in the mud? They're about 20 pounds. Do a quick bit of math and you'll see that the Iowa's main gun round is 135 times as heavy as an Abrams' main gun round. The Abrams may be the world's toughest tank and can take a ton of abuse, but not this level of abuse.
The Abrams can withstand a lot, but it'll lose once it is hit by a 2,700-pound shell. (DOD photo)
To put it simply, a main gun round from the Iowa-class battleship will destroy the Abrams easily. In a way, this speaks well for the Abrams – one can't really imagine anything short of an Iowa's main gun being able to destroy one.