Metro-city populations are such a fascinating topic. However at the rate the mid-atlatic and northeast US are growing, I think there will soon be a #1 city of Washington-Baltimore-Philadelphia-Delaware-Patterson-Newark-New York-Boston area with a population of 41,100,000. And of course it will be known as The Sprawl (eventually extending southward to Atlanta, but I think that will take a while longer...).

This page lists the 101 largest metropolitan areas of the world by population. This is a controversial issue rather difficult to quantify. These figures include suburban areas immediately surrounding a major city and sometimes multiple major cities which may be close enough together to function essentially as one area (e.g. Washington-Baltimore).

[ List of metropolitan areas by population: Information From Answers.com ]

And for me personally the largest metro area in the world that I had never heard of before reading this is Kolkata-Howrah in the Indian state of West Bengal. Apparently the 3 most populous countries that I know very little about are China, India, and Brazil. All of the areas that I'd never heard of reside in these countries. Which also explains how Bengali and Portuguese get so high on the list of spoken languages of the world.

Update: Once I said Kolkata out loud, I realized it is Calcutta. So Tianjin in China is largest city I've never heard of.