One 40oz. can of any flavor you like (light, dark, stout), or a 1.5kg “tall” can of the same. The 1.5kg can contains more malt extract so you can make a larger batch or use the same method here to make a richer beer. You can also buy ‘pre-hopped’ extract, which will impart more of a hop flavor to your beer.
For even better results, consider using two cans of the malt extract and not using any sugar. This adds to the expense, but further enriches the taste of the beer.