If specced into Agonizing Poison
Mastery is by far your best stat as it buffs your poison damage, while haste is very far behind crit, making is your worst stat. Versatility and crit are relatively close to one another, but I've found that with NH gear, versatility is slightly the favorite between the two. It's worth noting that there's no magic number for crit, or any sort of a crit "softcap" you should be aiming for.
Having Mantle of the Master Assassin also drops the efficiency for crit by a bit, but not overwhelmingly so.
If you're specced into Alacrity versatility should be very close the mastery levels, making it almost as good, with crit slightly behind the two, and haste way further down. Overall, Alacrity shouldn't change your stats too much, and playing it with the same stats you'd play Agonizing Poison with it is reasonable.
If specced into Exsanguinate
Versatility is the clear favorite with a bleed focused spec, with crit and mastery having just about the same value and, again haste is much further behind any other stat.
Without Mantle of the Master Assassin:
Currently, the best combination for legendaries is Duskwalker's Footpads and Mantle of the Master Assassin. If you absolutely need the execute DPS, then replacing Mantle of the Master Assassin with Zoldyck's Family Training Shackles is preferable.
Duskwalker's Footpads: Following the cooldown reduction trend of the spec, these boots are your absolute best legendary. They synergize well with Vigor and help you shave off the last few seconds of your Vendetta to sync it up with Kingsbane.
Mantle of the Master Assassin is your second best legendary. It amplifies the damage of Draught of Souls by quite a bit while also helping the flow of the spec by making your Mutilate crit during its duration. Optimally, you want to pool energy before using Vanish. If Vendetta is about to come up, you may delay the use of your Vanish to stack it with your Mantle of the Master Assassin buff.
Zoldyck's Family Training Shackles is your third best legendary. It gives Assassination the ability to execute again, as it used to in past expansions through passive Poison and Bleed damage. It's extremely useful in situations where you need the damage sub 30% such as Star Augur and perhaps Mythic Gul'dan.
Insignia of the Ravenhold is a very flexible legendary that adds passive damage to your Mutilates that cleave any mobs in front of you. Its damage usually varies from 3% to 5% overall. It's a middle of the pack legendary that works well for every rogue spec and doesn't penalize you by being locked to one spec.
Cinidaria the Symbiote adds a 30% dot on your target above 90% with every attack that deals damage. It's not a very noteworthy legendary and has very little use as it becomes a stat stick below 90%. It works particularly well with Mantle of the Master Assassin.
The Dreadlord's Deceit used to be a great legendary in the past with fights like Helya mythic, but has fallen in favour quite a bit in Nighthold. It's fairly useful in mythic+ dungeons and it certainly still has some use when aoeing mass amounts of mobs. Its buff also works for Subtlety so it has that double spec potential in the very least.
Kil'Jaedens Burning Wish casts a meteor around you. Its an underwhelming legendary, as it takes up a valuable trinket slot and has very low impact for sustained damage. It could be useful if there are a horde of stacked adds, but is otherwise lackluster in every way.
Generally speaking, both 2 set and 4 set are fairly big dps increases and you should be trying to get your hands on that tier asap.
Nighthold Tier 2 Piece adds a passive dot on your target through your Mutilates. This dot will be up 100% on your target and shouldn't be tracked unless you're pooling for excess amounts of time.
Nighthold Tier 4 Piece buffs your Envenoms by 10% for each bleed you have on your target. This works with your 2 piece buff, Garrote and Rupture alike. It's something to keep in mind when multiple mobs are present and you have Garrote running on only one.
These are the trinkets you need to get your hands on asap:
Draught of Souls is a cast that lasts 3 seconds on a 1:20min cd. It works well with Mantle of the Master Assassin and while its value might drop a bit if you don't have that legendary, it's still a very useful trinket that ranks in my top3. Note: While you're in your Draught of Souls cast, you're still auto attacking, so it doesn't negatively impact your Kingsbane damage. You need to use this trinket right after a finisher to get the benefit of Surge of Toxins. This trinket does not stack with Vendetta.
Nightblooming Frond comes second to Convergence of Fates for me. It's a very inconsistent trinket that adds an extra layer of RNG to the spec but can be very powerful on a lucky pull. It's a good option to use over Draught of Souls if you do not have Mantle of the Master Assassin.
Great trinkets that you're not likely to find at high enough ilvl to warrant replacing any of the above:
Ethereal Urn/Eye of Guarm(Mastery) I'd pick this over Nightblooming frond if at same or 5-10 less ilvl solely due to its consistency.
Entwined Elemental Foci & Arcanogolem Digit are the other two trinkets dropping in Nighthold. They're considerably worse than the top 3 and shouldn't be used unless they happen to drop at a high ilvl.
For Agonizing Poison builds:
As always, make sure to use Defiled Augment Rune to boost your dps!