If we're talking about singing on their most recent works, then I would say that Mustaine does indeed beat Hetfield.
I mean, compare the System has Failed vocals - for all their unspectacular nasalness - to St Anger (fran-tic-tic-tic-tic-tic-tic-tic-tic-tic-tic-tic-tic-tic-tic-toc) = GODAWFUL.
And as for Mustaine being a pre-pubescent Macaulay Culkin, yeah fair play, but Hetfield hardly sounds the mature one whining "I'm madly in anger with you".
I think the conclusion to make is simply this - not a great singer, fitted well with their early material. End of.