What was the most effective weapon in Saddam's arsenal?

His tank battalions got their butts handed to them, their infantry didn't do much better. Their air force was a joke. Only things that seemed to cause a lot of casualties was their artillery. Would you consider that their best weapon?

Then of course later on houses... since we had to go house to house fighting which caused a lot of casualties. However those and IEDs can't really be given to Saddam for credit as some of those groups killing us were anti-Saddam Jihadis. But even his own military in the bigging used houses and towns to great advantage to cause casualties on the US side.