What types of weapons should not be legal?

We spend a lot of time arguing about types of guns and magazines. But what types of weapons should we not allow? If we’re to protect ourselves from our fellow citizens (and a possible tyrannical government), where should we draw the line (if at all)? And what reason would your give for drawing a line?

In other words, would you draw the line at surface-to-air missiles, biological weapons, nuclear, grenades? What would be your reasoning for drawing that line?