There is lot of different specs that are viable as sub right now. Depends on comp and personal play style. You just have to have the gear that compliments how you are going to play.
Tbh, you could probably play pve single target specs as sub in arena and run at people like assassination as Kyrian with relatively high success because of the dmg OR necro and play very close playstyle to assassination.
Caveat is you need to have an edge of night off hand from raid and every domination socket you get with the 1.5% dmg buff makes it better. However even the LFR version of edge of night
If you do pvp and pve sub is going to be the easiest to switch between contents types without gear swapping too much.
Stacking mastery is going to be highly beneficial and going and getting pve gear for mastery is better than the pvp gear with vers for sheer dmg. (3X more effective.) You will be slightly more squishy, but you can add of subtract vers depending with pvp gear depending on how tanky you want to be.
MA is still completely viable in all content for legendarys if you play hit and run style and M+ especially. Kyrian legendary does dmg at different times than MA. The thing that makes it deadly is that it does does dmg randomly so people cant do things like shield wall it or pain sup or BoP incoming dmg. They just get crit by a charged evis and die.
Completely unpredictable with a low CD and the move itself can hit pretty hard also and several of the charged anima point crits in a row and someone just gets erased.
Venth MA is a lot of front loaded dmg, and completely viable also, BUT less dmg if you dont kill in an opener with a long CD. (Not going to happen against most good players. It’s also getting a crit nerf in 9.1.5 which hurts it a lot.
I personally am playing a double dance, DS spec for biggest evis, but still have the utility. It works really well with the Kyrian charged anima point system for huge crits albeit at random times. And if thats not enough dmg you can always play single dance specs which is pretty much a guaranteed kill every 60 secs without any cds going out from your opponents.
Single best improvement as sub is getting an Edge of night. Even an LFR edge of night sims better than a regular 252 offhand for me and people are suggesting it still might be good next tier. That one item will make the biggest improvement in the spec across all content for you. So go get an even an edge of night from lfr.
If you ever see an instructors divine bell trinket in WQ or your command table be sure to get it. Biggest evisc you will see in game.
Check out my Youtube channel: Kodash OG (my forum account wont let me post links)
I also stream on tue and thur nights and sun afternoons if you want to watch and ask questions so you dont have to waste time try bunches of stuff that work for your playstyle.
And if you are brave and dont mind f bombs and trash talk. I answer real questions on my twitter. <@kodahsOg>
TLDR: Sorry for the wall of text but its all relevant with no bias. Gotta read it if you are interested or just starting. If not, just know sub has lots of viable options right now.