As many told above, SM/Ruin has the highest possible dps.
However, as your gear gets better, threat will rise as a huge problem. We had this problem back in SA, so we figured out this would work:
http://www.wowprovider.com/?talent=11226005_9_8r235c201525d5005350005002000100You lose at least ~25% dps from the SM/Ruin build, but after a certain gear level, it just doesn't matter. It will be all about threat. Sad, but you don't have anything better than this to deal with it.
Also to note: You need crits for any specs to work. Get as much as you can. More crit --> more +20% shadow damage debuff --> more love from the shadowpriest.
if you're bored with any of these, just go for the "full fire destro" build
