Use table sugar if that is what it asks for. I haven't used corn sugar for a couple of years; I think it is a rip-off, and there is no benefit from using it.
Depending on the beer, using malt extract in place of the sugar may improve the beer. malt extract will add more body, and give a 'fuller' beer. It will also add more non-fermentables and result in a higher FG.
It all depends on what the recipe is, beer is, and what you want, but generally. adding extract to basic recipes will be the best direction.
Any light extract will do; use the lightest you can get (Light, Extra light, Pilsner, Pale, etc).