For defense, use artillery or stealth tank. The stealth tank has short range but you can get behind the enemy unseen and then circle around him while you keep firing. Also good to break tank blockades, blowing up harvesters or just generally being a pain in the butt. If there are no base defenses, the stealth is also good against buildings. It does a lot of damage very quickly (it's not weak as some people think, it just has very short range so the missiles sometimes blow up before they hit - which is hard to see if you sit inside the tank).

[ March 16, 2002: Message edited by: Devon ]