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Ground, Ghost is a big no thanks to me. Sure it might have some advantages, but I am being just a tad bit shallow and consider it to be a bit hard to design for heh.
A steel/ghost would be sweet. The typing would be immune to rapid spin. (if it ended up being at all bulky) Not to mention that ghost gives it resistence against fighting and normal doesn't even hit it. (TAKE THAT FAKE OUT! No sash killing helps.)
Ghost sounds pretty cool at the moment, but I could be swayed by the time the poll rolls around (meaning I'm on the fence ^_^). I just like the idea of being able to come in on random Close Combats and the like. Also, being about to U-turn without fear of ever getting Vacuum Waved or Mach Punched is nice.
Removes Fire weakness, gives it a handful of new resistances, and doesn't give any new weaknesses. It also would make sense for a Steel/Dragon to have Levitate, which would remove a Ground weakness.
This is a tough one, but I'm going to have to say Grass. The resistances that it allows are too good to ignore, and if not, ground is my second choice.