Ultimately? Myself. My soul and my pride. Anything else - be it material possessions or my position as second-in-command of the Decepticons - could be taken away from me (provided anyone is strong or cunning enough, hah!), but I'll always be Starscream. And as long as my spark endures, I will not forget who I am, and I'll find a way to get where I belong (which is on top), even if it takes me a million years.
Much to my displeasure, this one has an obvious answer: Skyfire. I'd change his mind about not wanting to be a Decepticon. I still believe he would have it in him. You don't become a good scientist by being timid or afraid of the truth. He was good; one of the best. I could have shown him the truth, the truth of the Decepticons. He could have been one of us, us glorious creatures who know what we're meant to be.
He can fight, too - doesn't he fight for the Autobots? The Autobots! Those disgusting, weakling creatures with their idiotic compassion for fleshling worms. How I wish I could get every one of them and burn their repulsive red symbols from their bodies with acid! And Skyfire chose to fight for them? For them? Not for me, not at my side, for them? Is that all I was ever worth to him - less than fleshling vermin?
I'd like to kill them just for that. Any fleshlings, especially the Witwickys. Stomp on their weak bodies until their bones crack and they're nothing but bloody mush on the ground. And then, I'd get him. Pound some sense in the stupid, big white jet. No. I'd rip the lasercore right out of his body. Then I'd mount his head on my wall. Let's see how he'll leave me then!