Note1: special and super weapons may still hit your base.
Note 2: there are other ways to defend your base, i just think this is the best
one ;)

USA: build 3-4 fire bases at each entrance to your base. then put three missile
guys and a sniper in each. put 2 avengers behind each base, and keep a
tomahawk around to deal with artillery.

China: build bunkers at each entrace(build more than 2) and fill them with 2 Red
Guard and 3 rocket guys. then keep a nuke cannon around to take out enemy
artillery.

GLA: build a solid wall of stingers around your base, and put a tunnel network
somwhere so you can get in and out of you base. keep a rocket buggy or scud
aroud to deal with artilery.