Originally Posted by
jtstormrage
It really depends on the fight.
On Immerseus HC, Assassination is clearly best. You can do massive AOE damage on the adds (especially on 25 man).
It is the best spec on Garrosh if your raid needs output on parts 3 and 4 where Assassination really shines (the fight is all execute by that stage).
On some fights, Assassination is close to the other specs. On Juggernaut (the single target fight) it does comparable damage by the end of the fight (after execute with cds). It is also competitive on Nazgrim, Thok, Siegecrafter and Paragons (I find it very good on Thok where you can't Killing Spree during kite phase).
However, on fights where there is lots of cleave, Assassination is inferior to Combat by a long way (Protectors, Galakras, Spoils).
There are other fights where cleave is not a huge part of the fight, but Combat can do massive damage when it is possible to cleave (Sha during adds, Shamans at the start, Thok during bats, Siegecrafter during mines). Assassination really falls behind on those fights as well.
In summary, Assassination is ok, and it is certainly "viable". Combat is better overall (by a margin). Forget Sub - too much effort for very little reward.
PS you can optimise damage on add fights (Galakras and Spoils) as Assassination or Subtlety by using Marked for Death, which is very strong for those fights.