Well, if we go by votes (for lack of anything better at the moment), the top 10 would be:
Morbid Angel
Metallica
Slayer
Death
Suffocation
Possessed
Megadeth
Testament
Dream Theater
Dio
In roughly that order.
Atheist, Kyuss, and Xasthur are close behind. Kyuss's status as a metal band is under debate, I guess. I haven't heard any negative comments about Alice in Chains and Clutch (besides "not metal"), and I'm pretty sure both would rank pretty high on the list if their styles weren't so at odds with the general listener base here.
As far as the preponderance of death metal on the vote-based list, I can't really comment on Death, Suffocation, Possessed, or Testament. As far as Slayer, I'm only familiar with "Reign in Blood", which I don't think would single-handedly earn them a spot on the list (it's practically a half hour of the same set of musical ideas repeated over and over).
Now, if anyone can vouch for any of those death metal bands as being less creative than Slayer, then I think we can safely take them off the list. I'll have to leave that up for debate, though.