but when you say "riffing" what are you talking about mainly? Are you talking about writing memorable riffs that automatically becomes a signature style and sound for the guitarist or are you talking about who can play some seriously crazy stuff?
I mean, Jon Schaffer is good and has written some cool stuff but he's not awesome really. There is nothing that he does that I can't play pretty easy ... and I'm not trying to sound like I'm great or anything I swear, just saying that once you get the hang of fast repeating rhythm patterns its pretty much dick easy to play ... just gotta keep the endurance up
Scott Ian is a great choice ... but again, talking about memorable riffs. Holt has always been a favorite player of mine and has written some amazing riffs but nothing on scale with what Hetfield has done in that regard ... my opinion