I can't believe that nobody has mentioned Russell in this thread yet! He may not have the widest overall range, but he's got to be close. His lowest would be probably be the second verse in "The Turning" and I would say his highest would probably be the last line of the up-pitched verse in "Through the Looking Glass" (you know, the part where he screams right before the pre-chorus).
I like Midnight's voice, but his is not nearly as diverse and strong as Russells', so I can't give him the nod. King Diamond's voive is unique but all of his high pitch stuff is falsetto, so I don't really consider that to be an extension of his range. I'm not entirely familiar with some of these other guys, but the guy from Lost Horizon loses to Russell simply because of the cheese factor. My apologies to any Lost Horizon fans.
Anyway, maybe I'm off the topic, but I think Russell has an amazing range. Maybe not the widest in music, but close enough to where the strength and versatility of his voice win out over all the others listed. Of course, this is all my opinion.