Okay, this is tough because the Abrams, Challenger and T-90 main battle tanks are all pretty much worthy of commendation, so here's how it goes, in order...
Joint first place...
1 M1A2 Abrams (United States)
1 Challanger 2 (United Kingdom)
1 T-90 (Russia)
2 Merkava (Israel)
3 Leopard 2 (Germany)
4 Leclerc (France)
My personal favourite is the T-90. I have this believe that Russia has always produced the most reliable, most widely used and therefore, the 'best' military technology in the world.
From the AK-47 Assault Rifle, to the MiG Fighter Jet, RPG's, Dragunov Snipers and 'T' type tanks...You name it, Russia made it and the world's armies used it.
Russia may not of produced the most technologically advance weaponry, but technological advancement by no means constitutes how good a weapon is. After all... the more advance a weapon, the more things can go wrong.