To add to Gustavo's correct explanation, I think there is no contradiction about the use of "of all time." At first, it should be made clear that the grammatical concept of time differs from that in real life. "of all time" indicates a statement is true through the time that it is made. It may or may not include the real-time present moment, contingent upon the context. His being the best neurosurgeon of all time was true at the time that the statement was made, with no reference to if it is true at the time we are talking now. It is still valid if we say "he is considered to be the best neurosurgeon of all time." "of all time" happens to include the real-time present moment, even if he is dead. However, if we say "He is considered to have been the best neurosurgeon of all time," it may mean he is no longer so, or as Gustavo said, he is dead. Furthermore, if he was considered to be the best neurosurgeon of all time, clearly, the phrase "of all time" does not include the real-time present moment, but some definite time in the past when the statement was made.