There are many. The ones that really stand out are as follows.
James Hetfield, (for his vocals, and bad ass rhytm guitar chops)
Dave Mustaine, (he's amazing, one of the fastest guitarists I've ever heard)
Angus Young, (originality, stage antics, unbelievable skills)
Dimebag Darrell, (for his thick, heavy, balls to the wall, tone on VDOP, (did I mention his sheer fucking shredding awesomeness?))
and lately
John Petruci, for his technique, he's plain fucking awesome.

Probably forgot a few. If they read this and get mad, I will appologize.