I'm sure there are better bass players,

But here are my favorites (and therefore the best, in my opinion):

- Geezer Butler

And the highly, highly underrated:
- Peter Steele
and
- David Ellefson

And the relative unknowns:
- Martin Mendez of Opeth - seriously guys, you all HAVE to hear this guy,
- James Sheppard of Nevermore - being the bassist of the best metal band ever, makes him one of the best bassists ever.

I can't believe no one has mentioned those Steele or Ellefson. Peter Steele is not only a fantastic bassist, but an amazing singer and songwriter.

Dave Ellefson is one of the most underrated bassists around. Maybe it's because some people see him as Dave Mustaine's sidekick (I certainly don't). He's always kept up, note for note, with Mustaine's psychotic rhythym guitar. That in itself warrants him being on any best bassist list.