First, let us look at individual members of the bands.
Tom Araya vs. James Hetfield-
The vocalists for the bands. Araya for Slayer and Hetfield for Metallica. Some say that Araya lacks range to his ability, but he can still go through an hours-long concert screaming his voice out for Slayer’s songs while I sincerely pray you haven’t had the torture of listening to Hetfield sing, if you can call it that, live. It’s horrible. I heard some of my favorite Metallica songs once done live with him and I was ready to kill him. Even moreso than when I was reading about how much of an ass he is.
Kerry King/Jeff Hammerman vs. James Hetfield and Kirk Hammett-
This is one I can’t decide. King and Hammerman are extremely talented guitarists but Hetfield and Hammett made some of the best riffs ever. I would argue some of King’s and Hammerman’s bigger and more well-known songs like Angel of Death, South of Heaven show their immense aptitude for guitaring that could rival Hetfield/Hammett. But hey/Mustaine/Friedman might take them both.
Dave Lombardo vs. Lars Ulrich-
Now this is just a slaughter. Lombardo is one of the best metal drummers alive. It’s like comparing Neil Peart from Rush to Joey Jordison from Slipknot. Besides, no one likes Lars.
Tom Araya vs. Cliff Burton-
Yeah, this is a clear winner sadly for Metallica. Araya is not the greatest bassist and I don’t think he’s ever attempted to be.
Now, if we were to compare which album by the band is considered most influential and legendary, it be Reign in Blood compared to Master of Puppets which is not entirely fair. Reign in Blood has some fantastic riffs but compared to MoP it just won’t cut it. Let’s try Seasons In The Abyss vs. Master of Puppets. That would be the title track, War Ensemble and Dead Skin Mask vs. the MoP title track, Leper Messiah and let’s say....Battery. I personally thought that all six songs were fabulous but I honestly would much rather listen to Dead Skin Mask than Leper Messiah. It impressed me far more.
Finally, the bands generally. Some people I’ve noticed are fond of saying "old Metallica kicks Slayer’s ass." There is no old Metallica. Old Metallica is not a separate entity from new Metallica. They’re the same band and that band has degenerated into absolute shit. Compare how the bands have progressed with God Hates Us All vs. St. Anger. NO competition whatsoever. People have to face it. The good Metallica died long ago and Slayer has been kicking major amounts of ass for their whole career even when they lost Lombardo for a couple albums. And don’t tell me that "well, at least Metallica evolved." They didn’t evolve. They sold out for what sells while Slayer stays with the sound that brung them to the dance and shows us that even after 20+ years, they are still able to make that sound in a new way to absolutely own your soul.