The Nightblade can choose to specialize in a variety of weapon types and armor types, but for most purposes the dual wield skill line will be the most valuable as far as weapon skill lines go. This is largely due to the passives that scale directly with most of the Nightblade’s active abilities. Some dual wield abilities are not worth taking. Flurry is one example. Others like Whirlwind and Twin Slashes are quite decent, especially when paired with the more devastating attacks from the Assassination skill line.
While leveling, Nightblades will want to focus on both active abilities that deal damage as well as others that serve as an escape from combat and a way to disable opponents. Balance is key. At level 15 once the Nightblade is able to equip two weapon sets, they should incorporate both sets to give them a palette of abilities useful for any given situation. Using a ranged weapon such as a bow for a secondary weapon isn’t a bad idea.
The Assassination skill line will have most of the direct-damaging attacks that will take out opponents quickly. The Siphoning skill line also has a few utility abilities that are worth taking. Strife, for example, is good early on since it’s a health leech that also does damage over time. Drain Power also isn’t bad if you find yourself low on Stamina and Magicka often.
As far as armor goes, the leveling ESO Nightblade will probably want to stick to medium armor. This will give them extra critical damage, attack speed, Stamina regeneration and useful bonuses to sneak. Make sure to equip a full set of medium armor and place skill points into the skill line to make use of these passives.
Nightblades who pick their skill lines smartly and choose a variety of active abilities that boost their damage as well as give them plenty of utility will be able to level quickly and pick and choose their targets with care. This will give them an advantage over most other soloing classes. Since leveling in ESO largely revolves around soloing capabilities, the Nightblade will be one of the most eso efficient leveling classes.