Best tank is a hard thing to classify. In terms of importance, I have to give a three-way tie to the T-55, T-60, and T-72 for being exported and copied, and their influence in many conflicts.
For longevity, arguments could be made for the Centurion, Sherman (with Israel mainly), and T-34's (served in Angola in the 80's).
As for best right now, I have to say the M1a2 Abrams, followed closely by the Leopard 2a5 and Callenger III. The other "Western" tanks follow closely behind, with the T-80 series in the middle.
When you start looking next-generation, I say don't bother, who knows what who is cooking up? What flaw or strength might not reveal itself until tested in battle?