Your question is too poorly defined to answer. The diversity of form included under the term 'prehistoric humans' is significantly larger than that covered by living humans. Who are we looking at? The Vindija Neanderthals? 'Mungo Man'? The Idalto skull? These would all have looked very different to each other, even without taking into account features we can't directly observe from fossils such as skin and hair colour. The difference between the three would quite likely have been noticeably greater than the differences between living groups of people.Please explain & fill in details regarding your opinion & knowledge. But please do answer the main question first before adding in more details, even if to explain how this subject statement has no merit!
We could try and figure out which modern racial groups share most features with individual fossils; but there's no meaningful answer to the question of which is most similar to all of them.