Are you talking just absolute, flat out, most technically talented bass player?
I'd nominate Victor Wooten, from a band called The Flecktones. They're not a metal band, they're a Jazz-fusion-prog-bluegrass band lol, very weird, but very cool stuff, and have perhaps the best bass player in the world. Certainly the best I've ever seen or heard. This guy plays faster then any other bassist I've ever heard, hell, he plays faster then most lead guitarists I've ever heard. He's like the Allan Holdsworth/John Mclaughlin of bass. I've seen him live, and while I had only heard a little but of his studio stuff, seeing him live really blew my mind. He often plays with all ten fingers, ripping both hands up and down the neck of his bass in a blur of 64th notes with astonishing precision, and mindblowing intricacy. It's really hard to comprehend how amazing this guy is without seeing him live. But for an idea, check out the album Victor Wooten: Show Of Hands. Listen to track 12 "Classical Thump", realize it's one guy, one take, one bass...and watch your bass-playing dreams crumble. Then realize his extended bass solos in the Flecktones live shows are well known for blowing that clean out of the water without even trying.
Also plays classical and classical style on bass, gotta love that.
I listen to a lot of music in a lot of genres, metal and non metal, and he's far and away the best I've heard or seen.
Other notables you may not have heard of:
Billy Sheehan ( Niacin, Session on tons of stuff )
Tony Levin ( King Crimson and Tons of stuff throughout the years, check out Bozzio-Levin-Stevens )
Trey Gunn (Also King Crimson, plays "Touch Guitar" an instrument that comes in 8-18 string varieties and is played with both hands on the monster fretboard, range of a grand piano, VERY low, fat tones. Check out the song "Construktion Of Light" for a quick example. )
Rick Laird ( Mahavishnu Orchestra, back in the 70s )
Chris Squire ( Yes )
John Myung (Dream Theater, amazing player, too bad he's so often lost in the mix, but see him live, and wow! )
Sean Malone ( Cynic, Gordian Knot, others )
Jonas Reingold ( The Flower Kings, Time Requiem, Karmakanic, etc. A lot of prog and power metal, but an Amazing player, an absolute fretless monster )
Some more metal related bass players:
John Vesano ( Formerally of Nile, blew me away live when you can actually see the basslines being played. Very fast and complex stuff, duh )
Eric Langlois ( Cryptopsy )
Martin Mendez ( Opeth. Or is that Lopez? I always get the two Martins mixed up. )
Tyr ( Borknagar )
And Mark's pretty damn good too!
Now alot of the guys I mentioned are in bands with some pretty heavy progressive leanings, or jazz leanings, but whether you love or hate that stye of playing, it has to be admitted those guys are GOOD! And if you're talking sheer talent, I'd put them up there with many of the best.
I'm sure I'll think of many more as soon as I post, lol.