Cyanide
It had been playing on his mind for a while by the time he'd driven off and left Sam behind in Blue Earth. Though he'd never admitted it to anyone, and was ashamed of how weak it must make him in the eyes of Sam and Bobby and Cas, for weeks he'd been considering going to find Michael and just saying yes. And now at last, he'd gone and done it. Just got in the Impala and gone.
He had the radio on and amped up to full volume, blasting angry noise from the Impala's speakers to stop the silence creeping up on him and tempting him to have second thoughts, but it wasn't enough to disturb the strange, almost eerie sense of peace that had settled on him now that he'd finally made up his mind.
There was no more delaying the inevitable. No more fighting an impossible fight, because he's seen where this road will lead, and now he's about to ask an angel to come and end it all. There was no light at the end of the tunnel for him. There was nothing Dean could see that made it worth trying to resist it any longer, and he was ready to take whatever way out was offered. Famine had been right: he was already empty. Living dead inside. Dean was tired, and he wanted his peace at last.
Maybe outright suicide wasn't a card he had on the table, but saying yes to Michael was close enough.