Find the largest number n positions away from n
05AB1E, 21 bytes

vyN+Ny-})¹gL&lt;Ãv¹yè})Z


Explanation

v      }               # for each num in input
 yN+                   # push index + num
    Ny-                # push index - num
        )              # wrap stack in a list
         ¹gL&lt;Ã         # remove indices outside the range of input
              v¹yè})   # get list of elements in input at the remaining indices
                    Z  # get max


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