Who is the Greatest hero of all ages?

[Voronwe]

The Second Prophecy of Mandos in HoME V predicts that Turin will slay Melkor at the end of time.
However, this was only in an earlier version of the Silmarillion, and I think Tolkien changed his mind about it (and the whole idea of the prophecy) later on, since it does seem rather strange and out of place to have Melkor killed by Turin. Nonetheless, although it was rejected, it's an interesting piece of writing:

Then shall the Last Battle be gathered on the fields of Valinor. In that day Tulkas shall strive with Morgoth, and on his right hand shall be Fionwë [i.e. Eonwë], and on his left Túrin Turambar, son of Húrin, coming from the halls of Mandos; and the black sword of Túrin shall deal unto Morgoth his death and final end; and so shall the children of Húrin and all Men be avenged.

-Voronwë