It really depends on the style of metal you are playing, as metal has so many sub-genres at this point. It'd be the equivalent of someone asking, "what's the best bass for popular music?"
When I think of modern metal sounds for bass, I think a Dingwall played through a Darkglass pedal, i.e. Nolly from Periphery.
Old-school metal? A slightly fuzzy, overdriven signal from a P bass or J bass i.e. Geezer from Black Sabbath.
Honestly I think a great 5 string will get you where you need to go. I love 6 stringers, but unless you have a very hot output pickup, the high C string kind of gets lost in the mix. I think these 5ers will work in most cases:
EBMM Stingray 5 (HH or H model)
Lakland 55-94 or 55-02(people might argue on this one, but I've used it before for some down-tuned Mudvayne and it killed!)
Any Dingwall
Ibanez BTB, or Soundgear (I played a SR2605 that was just killer!)
Fodera (Ryan Martinie now has his own custom model)