Unpainted metal. Despite having enough unpainted lead to cover a church roof, I always keep an eye out for other figures - and then buy it. Why? I have unpainted Romans, Gauls, Parthians, Greeks, Dacians, Thracians - as well as two HOTT armies. Then the Napoleonics - unpainted Prussians, British and French.
Then there's the WW2 armour. Entire Divisions of US, Russian, German and British trucks, armour, troops. I think I've got more Russian armour than Stalin had.
So logically I'd avoid buying any more figures until I'd made a dent in the backlog, But no! See something you like and immediately the twitch starts. I saw some Vikings and Saxons in 6mm on E-bay and was bidding for them before I'd even considered if a) I'd get around to painting them b) if I'd ever get them on the battlefield and c) the wife would mind me spending another £20+ on stuff I don't even need.
Please make it stop!