I'd say Wlad simply because of his pure size. You can't train or supplement for height and arm reach.
In that case Haye cuz hes more Mike Tyson sized and a prime Mike Tyson kills the **** out of Vladimir Klitchko. Not that that really means much at all.
How can you know without doing a series of tests?? Yes Wlad is taller, but who has better power to weight ratio? Who has a higher Vo2 max? Who has faster reflexes? Who has better spatial awareness? Who is genetically more reactive to physical training etc? And then you have the things that may be genetic or may be learned such as heart or the ability to stay composed under pressure. And then you have to know if these attributes were trained or natural as if trained it has less to do with gentetics and more to do with completely separate factors. In summary there is no way for us on ESB to know! All you'll get is a load of Klit or Haye fans putting fingers in the air talking ignorant b*llsh*t and it will eventually end as a Vitali beat Lewis thread.