I recently decided to write a book about the M-4 and why I believe it to be the best all around tank of the war. I want to compare it to contemporary tanks from the German, Soviet and British vehicles. While deciding that the Panzer V or Panther was the best tank for the Germans, and that the T-34 was easily the best Soviet tank of the war research has shown a stunning array of British tanks. None of them seem to have been particularly successful and few were built in any numbers.
If I had to guess I would say that the Valentine was the best British tank of the war. Aside from the very late war Comet. It was produced in large numbers, had a large number of variants, saw service in many theaters and in many armies. And looked very cool.
However I am not an expert and would really like the input of someone who knows a lot more about the subject than I could learn by a perfunctory glance at tanks encyclopedia.