M1A2. Hands down. There really is no EQUAL. And I am NOT saying that because I'm American. I'm saying it because it is true.
But the British Challanger is damn good, and its armor (sorry, I mean armour) is made of the same composite the M1's is, just in a different ratio of what-to-what. The recent Leapords of Germany are hot too. And the Israeli Mkerva is in a class by itself. The Israeli armed forces have to meet a somewhat different mission profile than most western nations and thus their Mkerva while not being full of all the frills as some other tanks out there is first tier in its profile.
But seriously. The M1 series. Hands down. I'm sorry, but it's true. We have a hard enough time destroying it ourselves with our damn Hellfire missles. In fact, such a missile system is more than likely the only way you'll take that thing out right away, first try. You can blast the hell out of an M1 with most known anti-tank weapons system and not do much. You can make a reverse analogy to the Shermans and German tanks. 12 Shermans can blast away at a single Tiger while the Tiger easily picks off each Sherman one at a time (however like a German tank commander once said "It was the thirteenth Sherman that got you!" but that's a different story lol...)