I am having trouble deciding who is the most talented member ever to be in Metallica. However, I think if I absolutely had to choose, I would say Jason Newstead because he wrote/co-wrote and performed almost every song written by Metallica from 1988 to 1999. Lars Ulrich and Kirk Hammet are also very good, too.
Kirk Hammet is a God. He plays so smoothly for a Heavy Metal guitarist. His solos sound so sweet. He knows his instrument incredibly well, too. He could actually pass off as a blues guitarist... Well, maybe not. Another talented member is Hetfield. He plays difficult riffs while singing. It's crazy.
When Lars and James kicked him out, that was a good decision beacuase he went on to make Megadeth. Also, He gave Metallica the kick it needed to become famous AND when Kerry King was in Megadeth Dave taught him all about thrash and speed. Kerry King went on to make Slayer.
Dave Mustaine effected 3/4 of the greatest metal bands ever
You need to be pretty f*cking talented to do that.