Assassination has pulled ahead and now we feel that it is the best spec for Nighthold. Yazuki recently created an advanced assasssination guide for Nighthold here.
We are continually adding 7.1.5 changes and information under the 715 tag
What is the best DPS spec for Rogues in World of Warcraft's Legion 7.1.5 patch?
To sum up our thoughts: All of the rogue specs look relatively good. For the most part all 3 specs are within 5% of each other and exhibit varying different strengths. Additionally, we expect more changes before Nighthold is released and thus all discussion on the best spec is largely speculative.
So... You still want to know the best spec?
Well, the real answer depends on whatever Legendaries you have. That's it folks. Legendaries are so powerful that if you have a BiS for one particular spec it's going to outshine all of the other ones.
But, What if I have no BiS Legendaries? That's quite unfortunate.
Besides Legendaries another huge factor in determining your personal best spec is the trait level of your artifact weapons and the relics that are equipped.
If you have 54 points in Outlaw but only 20 in Assassination - you can guess which one will be best for you. If you have 3 BiS relics vs only 1?, etc.
Now you've read thus far and your probably still asking yourself: Assuming all gear, relics, legendaries, fight types, etc., are equal - which is the best spec??
Well hopefully you can understand why that is a silly question but we are going to go ahead and attempt to answer it for you.
At the start of Legion Outlaw was argueably the best spec for both raiding and M+. After the nerfs, Assassination became a clear winner in terms of the best raid spec. Now with all of the 7.1.5 changes and Nighthold what can we predict will be the best?
Well subtlety and Outlaw are both receiving decent buffs. Lot's of fights in Nighthold involve cleave/heavy adds thus making ST focused specs less powerful. At the start of Nighthold we are predicting Outlaw is going to be very strong. At the end of Nighthold Subtlety appears to win out. Assassination still looks to be a very good single target spec.
Update: With the recent changes on 7.1.5 (nerfing Greenskins again and the Sub 4 set piece bonus) -- and the fact that they are leaving the Assassination legendary boots untouched. We are now leaning towards the idea that Assassination is the go-to spec for Nighthold progression.
As we said at the start. They are all good. Hopefully you further understand why asking this question can be a bit silly.
As a final note:
It is unknown what spec will be the best in Nighthold. Final changes to abilities, set bonuses, and legendaries that may happen before or after the first week of Nighthold have yet to be publicized. Any discussion about spec balance in Nighthold/7.1.5 is largely speculative
Happy stabbing and may the RNG forever be in your favor!