I want to say heroin because with an unlimited supply of heroin being alone for 10 years wouldn't be so bad. But, at the end of that time I imagine I'd be using at least a few hundred dollars worth in a day. So, of course, I'd be completely broke and suicidally desperate almost as soon as I returned to civilization.
Then there's alprazolam; 10 years might as well be 10 days with an unlimited supply of alprazolam, but the memory loss would probably be permanent after that long, and again the withdrawals would be lethal.
So, after much deliberation, I'm gonna go with grade-A cannabis sativa. I could see just how much pot can be consumed by a single person in a single day. Hell in 10 years one could probably work up to a pound in a day (orally), but goddamn that doesn't even sound fun.