Q: What is the best race for Nightblade?
I always first recommend choosing a race which you like playing. However, there are racial bonuses to consider which in my opinion are not insignificant if you really care about squeezing every bit out of your character.
You can take a look at my comparison table of racial bonuses which will give you a brief overview of their usefulness for particular roles and builds.
As is the case with other classes, Nightblades can have many different builds and party roles, and no race is best at everything (except maybe Breton which I always like due to their all-around useful bonuses).
If you intend to go for the hardest endgame group content (Trials, aka raids), as a Nightblade you will most likely be either a tank or a mage (fire destruction staff). Imperials, Redguards and Orcs make for best tanks, and Breton, High Elf and Dark Elf best mages. If you wanted to swap between these two roles I’d recommend a Breton, as their magicka cost reduction on spells is useful for every build in the game.
If you don’t intend to do trials, then it doesn’t really matter which race you go for. Nobody will care whether you have 2% more or less DPS in Veteran dungeons, or in Craglorn group quests, so play whatever you like best!