"Best" probably varies from gun to gun. That being said, Remington, Winchester, and Federal will all have some similar Foster slugs. Brenneke has some varieties of slugs that might penetrate better due to their heavier weight and solid design.
ETA: I read once that Winchester, Remington, and Federal foster slugs had slightly different diameters, and because of that, people found different brands worked better with different chokes. While IC is generally recommended for slugs, some slugs actually worked better with MOD or even FULL chokes for some people.