Could depend on which Buster and which Smith. Both at there best, I'll plump for Buster, more versatile. Smith had a decent punch so there's a chance he could come from behind as he did against Bruno, but I'll stick with Buster getting a unanimous decision. But who knows with both these hot and cold guys.
Each at their best? I would say Douglas out boxes him clearly. Smith is a decent heavyweight but I cringe whenever someone tries to tell me he hit as hard as George Foreman or even close.
I don't actually believe in "Tokyo" Douglas. I just think he was a pretty consistent and talented boxer during that period, having beaten McCall, Berbick, and Mike Williams. Bonecrusher lost to basically every decent boxer he faced except the Witherspoon rematch, and Witherspoon had already beaten him. Douglas would win, same as Anthony Joshua.