You could say just kanapka jest smaczna or pizza jest pyszna (and in some contexts it would be enough) or more often use plural (meaning the general collective of such things): kanapki są smaczne, pizze są pyszne.
But it really depends on context, if you argue with somebody about the taste of a general pizza, either of those would be probably OK.
What silmeth said, plus "Kanapka jest smaczna!" can be translated either as "Sandwich is tasty", "A sandwich is tasty"or "The sandwich is tasty" depending on the context. But if you want to stress that this particular sandwich is tasty (Which in English you would do either by saying "The sandwich is tasty!" or "This sandwich is tasty!") in Polish you'd say "Ta kanapka jest smaczna!" ("ta" = "this")