>From a prog on the world's best tanks, over the years, I believe the American M1 Abrams comes out on top, with the British Challenger pretty close. Of WW II, of course: The Tiger and the Stalin.

Challenger didn't make this list, but the Centurion did. Also the M1 Abrahams came in 2nd, the German Leopard 2 came out on top.
http://www.discoverychannel.co.uk/greatest_ever/tanks/index.shtml