I'm a huge metalhead, and I like Zakk Wylde too, but I don't want to keep posting more shit from the same guy, so I'll mix it up a bit. I'm sure people are going to bring up the obvious names like Slash, Jimi Hendrix, Randy Rhoads, etc. One of the best metal guitarists/shredders out there is Marty Friedman. The dude is really incredible because he combines technical perfection with this dead-on sense of melody and passion in his soloing. Way too often you hear a shredder who might be amazing from a technical standpoint, but there's something cold and mechanical about his work - there's nothing in the soloing that really stands out. Not so with Marty; his technical mastery just amplifies his guitarwork, but it never overshadows it. He's also got an exotic signature sound that can't really be replicated by anybody else. If you listen to the work he did on Megadeth, especially Rust in Peace, you'll see what I mean. 3:07 for the solo. 0:00 - 2:30 and 6:07 - 8:18 for some of the most soulful, passionate guitar soloing you've ever heard. Another name that comes to mind is Glenn Tipton from Judas Priest. The guy is not just a guitar god, but unlike a guy that can whip out a really tasty solo, this dude is also a brilliant songwriter. Priest's material from the 70's stands as probably the best metal ever created (specifically, Sad Wings of Destiny, Sin After Sin and Stained Class), which is saying a lot considering they have a ton of classic albums released after that period as well. He stands out as probably my favourite guitarist ever along with Tony Iommi from Black Sabbath. The whole song is amazing, but it crescendos into one of the greatest solos ever written 3 minutes into the song.
Robert Fripp used to shred before it was cool. One of the most excelent sweep pickers I've seen. This guy does some pretty interesting stuff with the guitar (like play it with a dildo) but I can't stand his lyrics. Freak Kitchen would be awesome if someone else wrote the lyrics.