The stat priority
It's Str > Crit > Haste > Mastery.
No really, it is. Well, it is sometimes. It's also Str > Haste > Crit > Mastery, as well as Str > Crit > Mastery > Haste.
Here is a list of stat weights for unholy. "Base" has 20k str, 10k haste, 10k mastery, and 10k crit.
Crit has 20k crit, and otherwise the same stats, while haste has 20k haste and mastery has 20k mastery. The Str has 25k str. These profiles represent, at a basic level, gemming and reforging towards a particular stat, and how it changes the stat weights.
As you can see, the profile that goes towards haste, strength, and mastery all have crit as the best stat, but when you actually go towards crit, it becomes beaten by haste.
In general, at a base level, the priority remains Str > Crit > Haste > Mastery, and if you insist on gearing towards a singular stat, you'll get the best results with that stat priority, but for those of you who would like to maximize damage even more, we have a better option.
The goal for unholy is now to get all of the stat weights balanced. You want Crit = Haste = Mastery, and it is from that balance that you will obtain the highest dps. You'll also want those stats *2 to equal strength.
The question is, how do we obtain balance? Luckily, for the most part stat weight interactions are pretty simple. When you get more X, everyone other than X becomes better. So you want to add to X, when X is your best stat, until X isn't your best stat, and then add to Y instead.
Now, while you could do this by constantly regemming/reforging and importing your new build into simC, that is expensive and takes a long time. Luckily, there is a better way. You may have noticed that, below your simC profile is a set of lines. I'll give you the ones for my profile.
These lines are a summary of your current stats from gear. If you take the 4 important ones, and remove the #(which comments it out and makes the sim ignore it), you have easy access to edit your stats. Like so:
After you have that, and you've run your stat weights with your initial gear, add some amount to your highest stat weight (and take it away from your lowest), and then resim. Guess and check is the best method I've got right now, but I'm working on a calculator where you just plug in your stat totals and it finds out the ideal distribution of stats.
Here is the closest I could get after an hour or so of trying the guess and check method.
That's with 23k str, 14k crit, 8.5k haste, and 2k mastery.