If you're bored...maybe get a list to the people you know that know metal and ask them to list their top 20. You can word it how you want. Most Influential...Best Ever...Best since 1990...whatever.
But I would definitely label it. Because when you have such a broad scope of bands over 40 years and when you take into consideration how much metal has evolved and how many sub-genres there are...it totally clouds the voting if the poll is too vague.