Tyson. He'd just about completely matured physically and was around 25lbs heavier than the young Cassius Clay who was still filling out.
The only chance that Ali/Clay would have ever had at beating Tyson would have been the 1966 version of Clay that beat Cleveland Williams. And even that version probally would have lost.
If you're comparing 18 year-old heavyweights, then surely Hughie Fury was better than either of them. Unfortunately for Hughie, he has been well and truly overtaken since then. It will be interesting to see how things work out for Moses Itauma, who was also probably better than the 18 year-old versions of Ali and Tyson.
Tyson, without a doubt, at the age of 18 was more developed than Muhammad Ali in a physical sense and probably in a boxing sense as well...